14 BIOLOGICAL LECTURES. 



the living Q:g^ is "destitute of organization," but neither can 

 any one maintain that the egg-organization is identical with 

 that of the adult. Development is essentially a transformation 

 of one form of organization into another along the path of 

 cell-division and cell-differentiation ; and it is undeniable that 

 the adult form of organization is thus expressed earlier in some 

 cases than in others — for example, in the segregation of the 

 germ-layers in the polyclade, as compared with the annelid or 

 gasteropod. We are still profoundly ignorant of the nature 

 and causes of differentiation, and of its precise relation to 

 cell-formation ; and the question is probably not yet ripe for 

 discussion. It is, however, impossible to maintain that differ- 

 entiation in the Metazoa is entirely independent of cell-forma- 

 tion, when we recall the multitude of cases in which the lines 

 of differentiation coincide with cell-boundaries. 



