EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 51 



evolutionary development. We may expect, for example, that two animals 

 derived from a common ancestor will both retain some of the features of 

 embryonic development occurring in the life history (ontogeny) of that 

 ancestor. Furthermore, as noted in our discussion of Fig. 4.1, the more 

 closely related two animals are, the greater will be the proportion of their 

 ontogenies exhibiting similarities. Thus the human embryo and the 

 monkey embryo are similar throughout much more of their development 

 than are the human embryo and the fish embryo ( Fig. 4.1). 



De Beer (1958) states: "Similarity in ontogeny between any animals is 

 a proof of their affinity, and no evidence as to the adult structure of the 

 ancestor." The latter portion of the statement is intended as rebuttal of 

 Haeckel's version of the recapitulation theory but is a bit extreme since 

 after all embryos frequently do present some indication of the nature of 

 adult structure toward which they are developing. We can learn from em- 

 bryos something of the nature of the common ancestor in question. This 

 matter will become clearer as we consider some examples from human 

 embryonic development. 



Recapitulation in Human Embryos 



We turn now to some of the salient features of human embryonic de- 

 velopment, emphasizing those which give evidence of recapitulation. 



Each human being begins hfe as a single cell, the fertilized ovum. This 

 was formed by the union of a sperm cell produced by the father with an 

 ovum produced by the mother. The size of the fertilized ovum is near the 

 limit of vision with the unaided eye. The first cell divisions with which the 

 fertilized ovum begins its development are much like those diagramed in 

 Fig. 4.2. As a result of repeated cell division a ball of cells is formed. This 

 is similar tp the blastula (Fig. 4.2) except that it is at first not hollow. As 

 shown in Fig. 4.3 a cavity soon forms, following which an outer layer, 

 the trophoblast, and an inner cell mass can be distinguished. At about this 

 stage the embryo "burrows" into (really digests its way into) the wall of 

 the uterus of its mother, where it comes in close contact with the latter's 

 blood. This blood supplies the embryo with food and oxygen and removes 

 waste products. The trophoblast forms the means of contact between the 

 embryo and the maternal blood stream and contributes to the formation 

 of the embryonic membrane known as the chorion. The embryo itself de- 

 velops in the inner cell mass. 



The inner cell mass soon becomes differentiated by the formation of two 

 cavities separated by a double layer of cells (Fig. 4.3C). The upper cav- 



