32 BIOLOGICAL LECTURES. 



back to certain blastomeres to the "continuity of development." 

 " In consequence of the continuity of development," he says, 

 " every older cell group must arise from a preceding younger 

 cell group and so finally definite parts of the body from definite 

 segment cells." A truer conclusion would be: and so finally 

 definite parts of the body from any cell you please. Continu- 

 ity of development no more explains the fact that the first 

 cleavage is dexiotropic, that the ectoderm is segregated in three 

 quartettes of cells, that the mesoderm comes from a definite 

 cell of the fourth quartette, that certain cells always give rise 

 to certain organs, than gravitation does. Likewise the " inter- 

 action of cells " which Hertwig and Driesch have invoked to 

 explain so many features of differentiation is in this case an 

 insufficient explanation. How can cellular interaction explain 

 the fact that from the time of its formation a certain blasto- 

 mere, e.g., the Urvelarzelle of Neritina, is peculiar in size and 

 histological character or that it grows rapidly and divides 

 rarely, whereas an adjoining cell, the apical, grows slowly and 

 divides rapidly .'* If it is meant that differentiation is the result 

 of the interaction of different material substances of the proto- 

 plasm which are more or less definitely localized, then there 

 can, of course, be no objection to this view. 



These are but a few of the many striking features of deter- 

 minate cleavage which are not at present explicable by known 

 mechanical causes. In the main one is compelled to refer 

 determinism in development, whether it be in cleavage, the 

 formation of organs, or the reproduction of specific and indi- 

 vidual characters, to intrinsic causes, that is, to the structure 

 of the germinal protoplasm, without for the present being able 

 to explain hoiv such protoplasmic structure is able to produce 

 such predictable results. 



Even Driesch, who represented very different ideas in his 

 earlier writings, has said in one of his later papers:^ "The 

 facts make it necessary to suppose that there exists in the 

 plasma-structure of every fertilized o^g^ of a bilateral animal a 

 polar-bilateral direction of its particles. ... In addition there 



1 Driesch, H., " Betrachtungen iiber die Organisation des Eies und ihre Genese," 

 Arch, fiir Entwicklmigsinechanik, 7M. 4. 1896. 



