MUSEUM OF COMPARATIVE ZOOLOGY. 195 



The uniform thickness of the zone would favor this interpretation ; on 

 the other hand, it is sometimes too broad readily to admit that expla- 

 nation. 



This body is the centre of the peripheral radiation, and corresponds 

 consequently to the centre of the deeper stellate figure. In the 

 latter, however, I have not usually succeeded in finding any correspond- 

 ing well-defined structure. As in the earlier stages, one sees only 

 an irregular area, often homogeneous, but at times apparently made 

 up of a small number of coalescing, not highly refractive globules. 

 (Comp. Fig. 55.) In only one case (Fig, 48) have I seen anything like a 

 sharply limited body in the centre of the deeper aster. In this case it 

 was of about the same size as the peripheral body, and like it appeared 

 slightly flattened in the direction of the spindle axis. Still, the outline 

 was less sharp, and the flattening less conspicuous. 



The impossibility of fixing with accilracy the absolute, or even rela- 

 tive, degree of advancement of diff*erent eggs at this period, deprives 

 the observations in great measure of the value they might other- 

 wise have. This would be especially perplexing were it not that one 

 is at liberty, when the evidence is so uniform, as it fortunately is in 

 this case, to use the observations of others, even though made on differ- 

 ent animals. Not that there is no direct evidence of which to make 

 ase, but simply that it is less complete than it would have been, had 

 the eggs been more transparent. 



The spindle at this stage is not always prominent. In some cases 

 (Fig. 43) it is with great difficulty that its fibres can be distinguished 

 from the radiate filaments. At other times, when the superficial pro- 

 tuberance and the central body of the aster are already differentiated, 

 Its limits are very well marked. 



Thickenings in its fibres are often not easily made out. Even where 

 there is a prominent elevation of the yolk and a sharply marked body in 

 the outer aster, the central zone of the spindle sometimes appears as 

 hardly more than a continuation of the granular protoplasm which, in 

 optical section, seems wedged in between the surfaces of the stellate 

 spheres (Fig. 43). On the other hand, .eggs in which the elevation 

 is less pronounced (Fig. 48) may present a clearly marked median zone 

 of thickenings, or even two closely approximated zones. Figs. 43, 48, 

 (and others not reproduced on the plates,) evidently exhibit slightly 

 different stages, although the eggs were deposited in one bunch, cer- 

 tainly not more than a few seconds apart, and were subjected to the 

 same treatment. The prominence of the stellar elevation, which might 



