86 Jenkinson, Germinal Layers of Vertebrates. 



These clearly are cases in which, at certain stages, 

 the blastomeres are interchangeable, their destinies as 

 yet not fixed ; in determining their fate other factors, 

 their mutual position or rather the influence they exert 

 upon one another and — as experiment is abundantly 

 showing — the conditions of their environment, come into 

 play. 



And what experiment has thus shown to be true of 

 the individual finds an obvious parallel in nature in the 

 formation, under the stress of varying internal or external 

 causes, of homologous organs from cells or layers of 

 unlike origin. In the one case as in the other the same 

 end is attained by paths that are diverse ; one of these 

 may be a recapitulation ; all cannot. 



On all sides then — and we have now examined the 

 morphological theories of germinal layers from every 

 possible point of view — the facts forbid us to see in these 

 elementary organs of the embryo that definite pre- 

 determination for the performance of certain ontogenetic 

 functions which the hypotheses we have been criticising 

 demand. The germinal layers are not sets of cells of 

 universally identical origin which necessarily and in- 

 variably give rise to certain fixed parts of the adult 

 organization, but merely convenient terms for the 

 primordia of the structures of the adult. 



Similarly constituted ova may and do — as we have 

 seen in the various groups of the Vertebrata, and still 

 more clearly in the spiral cleavages of Turbellarians, 

 Mollusca, and Chaetopoda — segment and gastrulate in a 

 precisely similar manner, and give rise to cells which in 

 origin are alike ; and this similarity in the segmentation 

 or gastrulation of the ova of related forms we may, if we 

 insist on retaining the word, perhaps still call ' recapitula- 



