426 HEREDITY AND DEVELOPMENT 



value is a function of its position." But the " position" has a 

 more than merely topographical connotation ; it means, as Prof. 

 E. B. Wilson says, " the physiological relation of the blastomere 

 to the inherited organisation of which it forms a part." 



But, here again, even when we recognise as fully as we can 

 {a) the importance of the initial inherited organisation, [h) the 

 influence of segment upon segment as development proceeds, 

 and (c) the continually operative influence of the normal en- 

 vironmental stimuli, we have still to confess that the process 

 of development remains very mysterious. 



The Antithesis of the Two Views. — The student who is not yet 

 clear as to the antithesis of the two views of development outlined 

 above should read Dr. Chalmers Mitchell's admirably lucid intro- 

 duction to his translation of Prof. Oscar Hertwig's Biological Pro- 

 blem of To-day (London, 1896). It concludes with the following 

 contrast : " Hertwig says that all the cells of the epiblast, hypo- 

 blast, mesoblast, and of the later derivatives of these primary 

 layers receive identical portions of germ-plasm by means of doubling 

 [quantitative or integi^al [erbgleich)'] divisions. The different 

 positions, relations to each other and to the whole organism, and 

 to the environment in the widest sense of the term, cause different 

 sides of the capacities of the cells to be developed ; but they retain 

 in a latent form all the capacities of the species. Weismann says 

 that the nuclear divisions are differentiating [qualitative {erb- 

 ungleich)], and that the microcosms of the germ-plasm, in accord- 

 ance with their inherited architecture, gradually liberate different 

 kinds of determinants into the different cells, and that, therefore, 

 the essential cause of the specialisation of the organism was con- 

 tained from the beginning in the germ-plasm." 



That differentiation may occur at very early stages is certain ; 

 that it has potentially occurred, although there is no visible evidence 

 of it, is also certain ; it seems to us difficult to interpret this without 

 the hypothesis of differential division. 



At the 2-cell or 4-cell stage of the development of the egg of the 

 sea-urchin, the cells are equipotential, for an isolated blastomere 

 (even at the 8-cell stage) may develop into a complete larva (Driesch). 



But a little later, when invagination has occurred, when two 

 germinal layers are established, the cells are no longer equipotential. 



