70 BULLETIN OF THE 



to be of considerable thickness, and to envelop closely the peripheral 

 layer of yolk corpuscles. An enlarged view of the same, given in 

 Figure 30, PL VI., shows the finely granular structure and the vesiculated 

 condition of the hardened blastema. It also shows how the blastema fits 

 over the yolk corpuscles, and dips down between them. It is to these 

 depressed regions that the boundary lines of the polygonal areas are due. 

 Figui'es 31-33 are enlarged views of separate yolk corpuscles with the 

 accompanying blastema. 



The protoplasm of the blastema has a very characteristic appearance. 

 In addition to the common character of being very finely granular, the 

 protoplasm is throughout finely vesicular or spongy. The latter charac- 

 teristic is especially marked in eggs heated in water to coagulate the pro- 

 toplasm, and arises, I think, in the following manner. The fat globules 

 described as filling the protoplasm of the blastema in the fresh egg are 

 dissolved in the alcohol used for hardening purposes, and consequently 

 leave in the protoplasm spheroidal spaces of nearly uniform size, which 

 constitute the interstices. A discussion of the cause of the division of 

 the blastema into areas will be found under general considerations at the 

 end of the paper. 



I have been unable, for the want of material, to trace the final changes 

 in the germinative vesicle. In the earliest condition of the deposited 

 egg that I have been able to procure there is a single central nucleus 

 (Figs. 28, 29 nl.)^ which is doubtless the descendant of the germina- 

 tive vesicle. This is the first segmentation-nucleus ; it is large, oval, 

 very finely granular, and surrounded by a spheroidal mass of protoplasm. 

 The latter is in immediate continuity with the network of protoi^lasm, 

 Avhich extends throughout the egg. The yolk corpuscles in the vicinity 

 of the protoplasm, which envelopes the nucleus, are much broken and 

 become successively smaller in approaching the nucleus, and at length 

 appear to merge into the finely granular protoplasm. 



In the succi3eding stage the central nucleus divides into two of equal 

 size, which occupy a sub -central position (Fig. 34, nl, nl'). These nuclei 

 have essentially the same character as the one already described. Fig. 36, 

 from a two-cell stage of another egg, shows one of the nuclei with a cen- 

 tral vacuole [vL). The yolk is rudely divided at the same time, and 

 having been previously arranged in radiating branched columns {Deuto- 

 plasniasdulen of Ludwig), now forms two groups of such columns (Fig. 34). 

 It is probable that each of the two nuclei is divided into two others, 

 and that each resulting therefrom is similarly divided, but I have not 

 seen the four-cell stage. The next stage sectioned is one with eight 



