30 C. M. CHILD 



tabolic rate, at least as regards the more apical regions, before 

 embryonic development begins. If the axial gradient has been 

 present during the growth period of the oocyte, we should expect 

 to find the specialization more marked apically than basally, 

 and this is apparently the case. Differential inhibition of the 

 oocyte, perhaps during the growth period, would probably pro- 

 duce more extensive differential modification of even the more 

 apical tegions. 



If this point of view be correct, there is no good reason for 

 maintaining, as I did in an earlier paper (Child, '00), that the 

 cytoplasm of the annelid egg is fundamentally no more highly 

 specialized than that of the sea urchin, nor is there any reason 

 for asserting, as some others have done, that the fate of cyto- 

 plasmic regions is as definitely determined normally in the sea 

 urchin as in the annelid egg. In the annelid, cytoplasmic 

 specialization of some sort apparently occurs to a larger extent, 

 particularly in the more apical portions, before embryonic 

 development begins, than in the sea urchin. The facts seem to 

 indicate, however, that this specialization of the annelid cyto- 

 plasm is associated with an axial gradient or gradients and is 

 progressive during the history of the egg. During the process 

 of rejuvenescence in early embryonic development (Child, '15 a, 

 Chapter XV) the annelid cytoplasm still retains its specialization 

 to a considerable degree, while the sea urchin approaches more 

 nearly equipotentiality of parts. 



Concerning the nature of this specialization we know little. 

 The various investigations on the effect of centrifugal force 

 have shown what I maintained earlier (Child, '06), namely 

 that it is not dependent upon the visible so-called formative 

 substances of the cytoplasm, but is a feature of the cytoplasmic 

 substratum, the 'ground substance.' If it originate in differences 

 in metabolic rate, as suggested above, we should expect to find 

 it primarily in the 'ground substance' not in the visible deposi- 

 tions and enclosures of the cytoplasm, which are secondary 

 features. The apicobasal metabolic gradient is present at the 

 beginning of embryonic development in the annelid, as in the 

 sea urchin, but in the annelid modification of the cytoplasm 



