278 GENERAL EMBRYOLOGY 



determining structure of the ovum is really that of the under- 

 lying protoplasmic ground substance, and that the arrange- 

 ment of the various formed substances coincides with this, is 

 determined by it in the first place, but is not always or neces- 

 sarily concerned directly in the later differentiations of the 

 ovum. The defects following removal of parts of the unseg- 

 mented ovum, or of blastomeres result therefore from the loss 

 of parts of this underlying structure, and not from the loss of 

 the formed materials or " formative stuffs," which in such 

 cases at least turn out not to be "formative." 



Lillie has shown that carefully graduated centrifuging 

 reveals the existence in the egg of Chcetopterus, of certain 

 regional differentiations of the ground substance, indicated by 

 the differences in the ease with which the granules of various 

 sizes, and other structures, such as parts of the mitotic figure, 

 pass through it. These regions are not otherwise visible but 

 Lillie suggests that since they are undoubtedly real they may 

 represent or mark in some way the primary organization of the 

 cytoplasm. 



According to this view of organization the term " organ- 

 forming substances" for the visibly differentiated substances 

 is a misnomer. For if these are removed to abnormal positions 

 within the cell they are then not related to the formation of the 

 same structures that they are in normal development. 



At present it seems difficult, though not impossible as we 

 shall see later, to reconcile these two views as to the real seat 

 of the primary organization of the cytoplasm of the ovum; the 

 balance of evidence appears to favor the conception of organi- 

 zation as a condition of the fundamental ground substance of 

 protoplasm. But in any event it is perfectly clear that the 

 cytoplasm is organized definitely. 



We should call attention in passing to the fact that many of 

 the results described above indicate that cleavage is not to be 

 regarded always as a developmental process of primary impor- 

 tance. Conklin has called attention to the fact that in Cynthia 

 the early cell boundaries do not always coincide with the limits 

 of the various kinds of cytoplasm. The determination of the 



