Extracts from

The Ontology of Fetal Development

Barry Smith and Berit Brogaard

September 1, 1999

Abstract

We believe that, with a combination of detailed biology and rigorous ontological analysis, it is possible to resolve the issue of when the human individual begins to exist. We lay down a set of necessary and sufficient conditions for being a human individual and we determine when these conditions are first satisfied. We conclude that the pertinent categorial change takes place around sixteen days after fertilization, and we consider a range of arguments for and against this conclusion.
 
 

0. Introduction
1. The Marks of Substance
2. The Marks of Relatively Isolated Causal Systems
3. The Hierarchical Structure of the Human Organism
4. When Does the Human Being Begin to Exist?
5. The Varieties of Substance Formation
6. The Development of the Foster
7. Alternative Thresholds
8. Twinning
9. The Concept of Niche
10. Is the Foster Connected to the Mother?
11. Is the Foster a Part of the Mother?
12. Stages in the Formation of the Human Substance
13. The Necessary and Sufficient Conditions for Being a Human Being:
      An Excursus on Siamese Twins
14. How to Refute Our Argument
 

0. Introduction

We shall help ourselves in what follows to the Danish term 'foster', which refers in neutral fashion to the human zygote, embryo, or fetus at the different stages of its development, and we shall focus on the question of the transtemporal identity or non-identity of the foster with the human individual as it exists after birth.
 
 

6. The Development of the Foster

Our aim is to establish the ontology of the process by which human beings are formed. To this end we will need to consider briefly the biological details of the development of the foster.

The story begins when an egg-cell, developed in the ovaries, is released into the fimbrated end of the fallopian tube (ovulation). The egg-cell, swimming free in the fluid-filled tube, is encountered by a sperm, and the latter initiates a process of penetration. (This is what happens in the normal case. In very rare circumstances a sperm cell might attach itself to one of the additional, much tinier cells which are also floating in the fallopian tube. These are the so-called 'polar bodies', deriving from eggs released at earlier stages in their development.)

Fertilization. The first stage in the process of penetration is the fusion of the membrane of the sperm cell with that of the egg. This results in the formation of a channel that allows the passage of the nucleus of the sperm cell into the egg. The male genetic material that is carried by this nucleus then fuses with genetic material from the nucleus of the egg-cell. The two nuclei come into contact in the egg cytoplasm, and shed their nuclear membranes. Each offers up one complete set of 23 chromosomes, and these two sets of chromosomes become entwined around each other as part of a process which transforms the egg-cell into a new joint product, called the zygote, an unusually large cell which has the same membrane as the egg-cell before fertilization.

Cell Division. Immediately upon formation the zygote begins to undergo a process of cell division. During the very first cell divisions (up to the eight-cell stage), there is no qualitative distinction between the cell that is dividing and the cells resulting from the division. The cells are functionally and qualitatively undifferentiated. Thus, each has the potential to produce a complete human being (each is, as the jargon has it, 'totipotential'). The cells are kept together spatially by a surrounding thin membrane (the zona pellucida), which was also the membrane of the egg-cell before fertilization, but there is no causal interaction between them. They are separate bodies which adhere to each other through their sticky surfaces. The cells are at this stage still floating free, inside their common membrane, in the fluid-filled fallopian tube, but they have begun to move towards the womb (uterus).

Formation of the Morula. Because of the limited space within the zona pellucida, a compaction takes place between the 8- and 16-cell stage (day 3). ⁽¹⁰⁾ As a result of this compaction, the inner cells divide faster than the outer cells which surround them. At this time tight junctions between the cells are formed; neighboring cells are connected by highly selective permeable membranes which are marked by channels through which signal molecules can be transported from one side to the other. The resulting bundle of cells is now called a morula (Latin for 'mulberry'). The morula is formed as the cells move from the fallopian tube and into the womb. 

When there are about 60 cells present (day 4), there occurs a clear differentiation between the so-called 'inner cell mass' and 'the trophectoderm' (an outer ring of cells). The latter functions henceforth as the surrounding membrane in place of the zona pellucida which disintegrates. In addition, pools of clear fluid which had accumulated between some of the internal cells coalesce to form a common cavity called the blastocoel, a body of extracellular fluid in which the inner cell mass thereafter floats. The two groups of cells, taken together with this fluid, are now called the 'blastocyst'. The entire blastocyst itself floats freely in the uterine fluid for about a day and continues to exist disconnected from the mother.

Implantation. Over the next week (days 6-13) there occurs a process called implantation (also 'nidation', from the Latin nidus: a nest, or niche). The blastocyst, on completing its journey along the fallopian tube into the uterine cavity, moves into a position where it is in contact with the uterine wall, to which it adheres via its sticky exterior. Cells on its outer surface then begin to grow rapidly in such a way as to disrupt the surface of the wall. These cells actively burrow into the deeper tissue until they have become completely embedded. The inner cells of the blastocyst are however still not connected to the mother, since they float in the liquid contained within the trophoblastic membrane. With the implantation of the blastocyst in the wall of the womb comes the formation from its inner cell mass of what is called the 'embryonic disc'. This consists of two kinds of cell mass: the epiblast, which will eventually give rise to the embryo proper and to parts of the umbilical cord; and the hypoblast which will give rise to extra-embryonic membranes and tissues.

Gastrulation. When the embryonic disc is fully implanted in the wall of the womb (day 13), it is able to grow and can begin to differentiate into the various tissues that will characterize the human being into which it develops. Only at this point does the disc begin to receive nutrients from the mother. Until now, only cell division has taken place. The morula has not grown in size compared to the egg-cell; rather, its constituent cells have become smaller. Now however the embryonic disc begins to grow, as the neural folds are formed and the primitive cardiovascular system begins to function. This stage, which begins around day 14-15, is called 'gastrulation'. It consists in a convergence of the epiblastic cells into what is called 'the primitive streak', a stacking of cells on the end of the embryonic disc which will develop into the spine. During the process of gastrulation (days 14-19), the epiblast differentiates into three cell types: the mesoderm (which will become the smooth muscle coats, connective tissues, blood cells, bone marrow, the skeleton and the reproductive and excretory organs of the fetus), the ectoderm (which will become the epithelia and the nervous system), and the endoderm (which will become the linings of the respiratory passages and of the digestive tract). In addition, some cells of the epiblast form the umbilical cord. Not all of the cells of the embryonic disc are going to develop into the fetus. Some of its cells (the non-epiblastic cells) will form the extraembryonic membranes (the amnion, the chorion and part of the placenta) and other extra embryonic tissues. 

The embryonic disc is now commonly referred to as the embryo proper, a term which is used to describe the developing foster until the ninth week after fertilization, from when it is called the 'fetus'.

A few days after gastrulation there begins to form the fluid-filled amniotic cavity in which the foster will float until the end of its term. This amniotic cavity is within the wall of the uterus, and as it expands it brings about a consequent contraction of the uterine cavity proper. Surprisingly, therefore, the embryo/fetus is for almost the whole of its development not, strictly speaking, inside the uterus (or womb, or uterine cavity); rather it is lodged within a specially created cavity inside the uterine lining. This multiple-cavity structure enclosing the foster provides a cushion against mechanical injury.

The placenta is a flat organ which supplies nutrients for the fetus during its development. It develops from the outer cell layer (from the trophoblastic cells) in the early embryo which fastens itself to the wall of the uterus from around day 21 and becomes anchored to the mother via a maternal portion formed by part of the functional layer of the uterine membrane. Together with the other fetal membranes (the amnion and the chorion), it disintegrates some moments after birth, when it is delivered through the birth canal. 

The umbilical cord is an organ of the fetus that penetrates the placenta via two large arteries which radiate outwards from the point where breaks through into the inner surface of the placenta and divide into small arteries that penetrate ever further into the depths of the placenta through hundreds of branching strands of tissue known as 'villi'. These villi cause a rupturing of the mother's blood vessels in their vicinity and are thereby bathed in maternal blood. The constant circulation of fetal and maternal blood and the very thin tissue separating fetal blood from maternal blood bathing the villi provide a mechanism for interchange of blood constituents between the maternal and fetal bloodstreams. However, it is normally not the case that there is opportunity for the blood of one to gain access to the blood vessels of the other. Rather, nutrients, oxygen, and antibodies diffuse into the fetal blood in the capillaries of the villi, and wastes and carbon dioxide diffuse out of these capillaries into the maternal blood circulation. (Compare the way in which oxygen is transmitted to fish via pipes which feed air into the water of an aquarium.)

Development of the Fetus. At about 40 to 43 days after conception the rudimentary brain at the top of the primitive streak begins to function. At the ninth week, the fetus has almost all human characteristics (except for the face and genitals) and it begins to show signs of specific male or female development. During the tenth week, the face and the genitals begin to develop. In the twelfth week, when the foster is nine centimeters long, it begins to move its hands and feet. Its movements increase around the sixteenth week, when hair also begins to grow and teeth are developed. At the twentieth week, the foster can suck and swallow and its body bends and stretches. From then on the foster continues to grow in size until, at the end of the fortieth week, it is born.
 

7. Alternative Thresholds

Given this simplified account of fetal development, let us return to the question of transtemporal identity. When does the foster first satisfy the ten conditions set out above for being a substance which is also a relatively isolated causal system? At what stage is the foster transtemporally identical to the human being it becomes after birth? We distinguish the following nine possibilities:
 

a. The stage of the single-cell zygote (day 0)

b: The stage of the multi-cell zygote (days 0-3)

c. The stage of the morula (day 3)

d. The stage of the early blastocyst (day 4)

e. Implantation (days 6-13)

f. The formation of the primitive streak (days 14-16)

g. Viability (e.g. day 140)
 

a. The zygote is a substance: it is a bearer of change; it persists through a time-interval; it is extended in space and it has spatial parts such as the nucleus, the cell-membrane and the filaments inside it; it has its own connected exterior boundary which divides its interior from its exterior and which connects the parts within its interior and thus distinguishes it from a mere heap or collection. Moreover the zygote is an independent entity in the sense that it does not require the existence of any specific second entity in order to exist. (Thus it can survive transplantation.) The zygote is moreover, like every other cell, a relatively isolated causal system. It is shielded by its outer membrane from causal influences deriving from its exterior; the events transpiring within its interior are subject to a division between stable and critical events; and it contains its own rudimentary mechanisms for reestablishing stability in cases of disturbance. But there is an obvious reason why this zygote substance cannot be identical to the human being which will exist after birth, namely that it is predestined to undergo fission, and this means that it will cease to exist almost immediately after it has been formed.

b. The second possibility is that it is at the stage of the multi-cellular zygote-bundle that the human being comes into existence. But the zygote is at this stage most properly conceived as a sticky assemblage of 8 or 16 entities rather than as a single entity. They are not one but many. Although they are surrounded by a thin permeable membrane, this membrane merely helps to keep the cells together in the spatial sense; there is no flow of nutrients or signal molecules from the outside to the inside of the membrane or from one cell to another, and the cell bundle has no stability-restoring mechanism of its own of the sort which is required in order for the whole entity to be a single causal system.

Perhaps, though, we can hold on to the view that the multi-cell zygote is already a human individual by arguing that some one cell within the bundle is privileged by the fact that it inherits from the original single cell the property of serving as the bearer of identity for the human being that is in process of development. The problem with this view is that it contradicts totipotentiality -- the feature in virtue of which each of the cells within the multi-cell zygote has the full potential to develop into a human being. 

To see the problem here, we must understand how differentiation works. Differentiation is the creation, from a mere aggregate of homogeneous cells, of clusters of functionally and structurally different types of connected tissue at different sites. In the case before us, all the cells maintain forever the same genetic composition (that of the original fertilized egg-cell). However, the very genes involved contain the programming for differentiation. This programming goes into effect in different cells in different ways, not because of any intrinsic features of the cells themselves, but rather as a result of the specific environments surrounding them and thus of the macroscopic structures which they together go to form. This surrounding context determines that some of the genes within each given cell become repressed, so that only some types of protein are made. That it is the environment surrounding a given cell which determines what kind of proteins will be formed (or 'expressed') by the cell can be seen from the fact that, if cells of a given type are moved artificially to a different environment where they are surrounded by cells of a different type, then they will begin to express the same proteins as the cells which surround them. Since, at the stage of the multi-celled zygote, no differentiation has taken place, it follows that there can be no cell or bundle of cells within the cluster which is privileged in virtue of some intrinsic feature which it might possess.

c. At the formation of the morula, too, differentiation has not yet taken place, and so the just-mentioned argument can be applied in this case also (as also in cases d. and e., below). Each of the cells of the morula still has the potential to become a human being. At this stage, junctions between the cells of the zygote are formed which allow intercellular communication by means of small signal-molecules. But the morula still fails to meet condition 10. for being a causally isolated system. That is, it does not possess mechanisms of its own to restore stability in cases of external disturbance. At best it must rely on the separate rudimentary stability-restoring mechanisms of its constituent cells. The morula is still, in our terms, a loosely connected cluster of cells. 

d. At the stage of the early blastocyst, the cells have differentiated into the inner cell mass and the surrounding trophoblast. The inner cell mass constitutes a single substance, rather than many substances, insofar as its cells together form a single connected whole with a common physical boundary. The inner cell mass also begins to constitute a relatively isolated causal system, but it still lacks its own internal mechanisms in virtue of which its several parts would in cases of disturbance work together as a whole to restore stability. The inner cell mass will differentiate into two further tissues and only one of them will eventually become the embryo. The other will turn into membranes and extraembryonic tissue. This is not in and of itself important for determining whether or not it is transtemporally identical to the later human being, for one may argue that the membranes and extraembryonic tissues are merely temporary parts of the embryo in much the same sense as baby teeth are temporary parts of the child. What is important, however, is that, following our account of differentiation above, it is not yet determined which parts of the inner cell mass are predestined to become embryonic cells. Thus the stage of formation of the inner cell mass at day 4 still does not seem to be a good candidate stage for the formation of the human being.

e. When the process of implantation comes to an end, the embryo can begin to receive nutrients from the mother and can begin to grow as an individual and to differentiate into tissues of different sorts which are recognizable precursors of neonatal tissues. However, as for the early blastocyst, so also here, it seems that the foster still lacks its own integrated mechanism for restoring stability, and so it fails to be a relatively isolated causal system in the sense at issue. An identification of the foster as it now exists with the later human being, however, faces the problem that the foster has entered into a condition of being dependent on the mother the mother for nutrients and oxygen as well as for temperature control and other forms of protection. Does this imply that the foster is henceforth such as to fall short of being a substance because it does not satisfy condition 6.? Certainly, if it is extracted from the mother it will almost certainly die through lack of nutrients and through lack of an appropriate protective environment into which it can fit. But this applies also, for example, to an Arctic explorer in relation to her winter ice station. It is known from the animal kingdom that premature offspring can often survive in external environments. A kangaroo foster, for example, is born alive at a very immature stage when it is only about one inch long and weighs less than one gram. After birth it uses its forelimbs to crawl up the mother's body and enter the pouch which is a pocket on the mother which opens forward and contains teats. The baby kangaroo grows in the pocket and gradually spends more and more time outside the pouch, which it leaves for good at the age of seven to ten months. The amniotic cavity, in which the foster develops upon implantation, is in some sense like a kangaroo pouch, though instead of being open (in the sense of having an aperture) it is a closed cavity. As the kangaroo foster is not rigidly dependent on one specific mother, but only on an appropriate environment with teats and so forth, so the foster is not rigidly dependent on the mother, but only on some appropriate environment, which might from a certain point be supplied by means of an incubator. This means that the foster is not dependent on the mother in the ontological sense of dependence that is involved, for example, in the relation between a smile and a face, or between an individual instance of color and some extended surface.

f. According to Ford (1988), the point where a human individual comes into existence is the time when the epiblast ceases to be a cluster of homogenous cells and begins to transform itself into a heterogeneous entity. He suggests that this happens with the formation of the primitive streak (around day 16). At this point, he argues, the cells of the epiblast have formed a whole multicellular individual living being which has a body axis and bilateral symmetry; that is, it is spatially oriented. The embryo's cranial axis and its dorsal and ventral surfaces come into existence, and the whole has acquired effective mechanisms to protect itself and to restore stability in face of disturbance. 

McLaren (1984), too, offers a defense of the thesis that it is the formation of the primitive streak which marks the beginning of the human being. This, he argues, marks the beginning of a human individual insofar as the boundaries of a discrete, coherent entity have now been formed. At this point the foster also meets the other part of criterion 8. to the effect that it is protected against outside disturbance by the jacket of cells surrounding it. That is to say, at the formation of the primitive streak, there is formed for the first time a bona fide spatial boundary which delineates the embryo spatially from the extraembryonic tissue. Before this point it is not determined which cells will become the embryo and which the surrounding membranes. In the case of adult human beings, in contrast, it is determined which parts (for example: nails, hair) can be removed without the human being ceasing to exist. In this respect too, therefore, the foster is from the stage of the formation of the primitive streak more like the adult human being than it is like the foster at earlier stages.

The tighter integration of the foster that arises with the formation of the primitive streak is manifested most importantly in the fact that twinning is from this point on no longer possible. If fission occurs just before the formation of the primitive streak, this will in almost all cases give rise to malformations (Siamese twins).

g. It has often been suggested that the human individual begins to exist at the point when the foster becomes viable; that is, when it can live outside its mother's womb. The argument is that because, prior to this time, the foster cannot survive independently of its mother, it is analogous to an organ of the mother, which can only exist and exercise its proper function within the locus of its proper encompassing environment. The problem with this view, however, is that the transition to viability does not in itself connote a transformation of one entity into another. The passage into viability of the foster may coincide with such a transformation process, but it does not necessarily do so. Rather, this passage may involve no genuine change in the foster, but may represent a mere Cambridge change of the sort which is expressed by propositions such as 'John is no longer beloved in South Africa', or 'Mary just ceased to be the tallest player in the team.' Acquiring stronger muscles is a real (though not a categorial) change, but the satisfaction of the viability criterion is not dependent on such physical changes in the foster; it may be satisfied, for example, through advances in technology.
 

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Notes
10. The times indicated here and in the foregoing are of course approximate only. Variations will arise, for example, according to where in the fallopian tube the egg-cell is fertilized.