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in many cases it is doubtful if even an experienced embryologist 

 could distinguish between them. This great generalization 

 appears to have been first reached by the embryologist von Baer, 

 who in 1827 discovered the mammalian ovum. It may be 

 illustrated by the chick and rabbit embryos represented in 

 Fig. 117. 



Von Baer's generalization contained the germ of what is now 

 known as the Eecapitulation Hypothesis, or, as Haeckel has 

 termed it, the Biogenetic Law, which states that every organism 

 in its individual life-history recapitulates the various stages 

 through which its ancestors have passed in the course of their 

 evolution. In other words, ontogeny, or the life-history of the 

 individual, is a repetition of phylogeny, or the ancestral history 

 of the race to which the individual belongs. 



We have already referred, in Chapter IV (Fig. 13), to some of 

 the earlier stages in the development of that primitive fish- 

 like animal Amphioxus (Fig. 118). Let us next inquire how 

 these stages may be interpreted in accordance with the recapitu- 

 lation hypothesis. The unicellular ovum (Fig. 13, I) obviously 

 represents the remote protozoon ancestors which were common 

 to the whole animal kingdom, and which are also represented 

 at the present day by independent unicellular organisms such 

 as the Amo3ba. The segmentation of the ovum into primitive 

 embryonic cells or blastomeres (Fig. 13, II VI) represents the 

 transition from the condition of the simple protozoon to that of 

 the protozoon colony, in which the individual cells, instead of 

 separating, as in the dividing Amoeba, remain together, but still 

 without undergoing any marked differentiation and division of 

 labour. The arrangement of the blastomeres in the form of a 

 hollow sphere, the blastula or blastosphere (Fig. 13, VII), with a 

 single layer of cells surrounding a central cavity, represents the 

 formation of such a protozoon colony as we see in the existing 

 Volvox (Fig. 11) or Sphaerozoum (Fig. 12). The process of 

 gastrulation, whereby the single-layered blastula is converted 

 into a two-layered gastrula (Fig. 13, VIII X), with primitive 

 digestive cavity (enteron) and primitive mouth (blastopore), repre- 

 sents the transition from the protozoon colony to the coelenterate 

 stage of evolution, the latter being still represented at the present 

 day by such forms as Hydra (Fig. 57), Obelia (Fig. 60), the jelly- 

 fish, the corals and all their numerous relations, which retain in 

 their organization all the essential features of the gastrula, 



