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r.4 THE CLASSICAL TRADITION 



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Hertwig interprets this fact of the specific distinctness 



of closely allied embryos in the light of the preformistic 

 conception of heredity. According to this view the whole 

 adult organisation is represented in the structure of the 

 germ-plasm contained in the fertilised ovum, from which 

 it follows that the ova of two different species, and also their 

 embryos at every stage of development, must be as distinct 

 from one another as are the adults themselves, even though 

 the differences may not be so obvious. If this be the case 

 there can be no real recapitulation in ontogeny of the 

 phylogeny of the race, for the egg-cell represents not the 

 first term in phylogeny, but the last. The egg-cell is the 

 organism in an undeveloped state ; it has a vastly more 

 complicated structure than was possessed by the primordial 

 cell from which its race has sprung, and it can in no way be 

 considered the equivalent of this ancestral cell. 



Hertwig puts this vividly when he says that " the hen's 

 egg is no more the equivalent of the first link in the phylo- 

 genetic chain than is the hen itself" (p. 160, 1906, b). 



If ontogeny is not a recapitulation of phylogeny, how is 

 it that the early embryonic stages are so alike, even in 

 animals of widely different organisation? Hertwig's answer 

 to this is very interesting. He takes the view that many of 

 the processes characterising early embryonic development 

 are the means necessarily adopted for attaining certain ends. 

 Such are the processes of segmentation, the formation of a 

 blastula, of cell-layers, of medullary folds where the nervous 

 system is a closed tube, the formation of the notochord as 

 a necessary condition of the development of the vertebral 

 column, and soon. "Looked at from this standpoint it 

 cannot surprise us that in all animal phyla the earliest 

 embryonic processes take place in similar fashion, so that 

 we observe the occurrence both in Vertebrates and In- 

 vertebrates of a segmentation-process, a morula - stage, a 

 blastula and agastrula. If now these developmental processes 

 do not depend on chance, but, on the contrary, are rooted in 

 the nature of the animal cell itself, wi- have no reason for 

 inferring from the recurrence of a similar segmentation- 

 process, inorula, blastula, and gastrula in all classes of the 

 animal kingdom the- common de-scent of all animals from one 



