S2 JENKINSON, Genuma/ Laj'ers of Vertebrates. 



The experiments which most immediately concern us 

 are those instituted by Oscar Hertwig, Driesch, and other 

 embryologists on the segmentation of eggs submitted to 

 pressure, and on the behaviour of isolated blastomeres ; 

 experiments which in the first instance arose out of a 

 critical enquiry into the pretensions of the Roux-Weis- 

 mann hypothesis of preformation. This hypothesis, as it 

 is hardly necessary to point out, is a modern resuscitation 

 of the famous theory of evolution which was destroyed by 

 Wolff more than a hundred years ago, and postulates a 

 necessary predetermination of definite parts of the ferti- 

 lized ovum, or of its nucleus, for the production of particular 

 organs of the embryo. Segmentation in this case is a 

 qualitative process ; all that it has to do is to separate the 

 already different parts, and so make manifest that structure 

 which was invisibly present before ; it is, as Roux phrases 

 it, a ' Mosaikarbeit' 



Now at bottom the theories which have been criticised 

 above are also attempts to throw back the structures of 

 the adult into definite predestined parts, not necessarily 

 of the unsegmented, but at least of the segmented ovum ; 

 they are in fact essentially preformationist,^'' and, more- 

 over, the logical outcome of that doctrine. For, if develop- 

 ment is strictly a process of 'self-differentiation' pro- 

 ceeding wholly from causes resident in each part and 

 independent both of other ' internal ' conditions — such as 

 mutual position — and of 'external factors' — such as the 

 physical and chemical composition of the environment — 

 then the potentiality of each part is predetermined and 

 limited, similarly constituted ova will develope through 

 similar stages into related forms, while any dissimilarity 

 occurring at any stage will lead to the production of 

 organs that are not homologous and forms that diverge, 

 and, since the prime cause of differentiation — the structure 



^^ This has also been pointed out by Braem {I.e. p. 435). 



