DEVELOPMENT OF FLUSTRELLA HISPIDA. 443 



the staining reagents employed exactly resembles that of the 

 chromatin granules of the nucleus. 



Shortly after this stage these extra-nuclear granules 

 coalesce, and eventually come to form a crescentic body — 

 the yolk nucleus — which (PI. 22, figs. 8, 11, 13) becomes sur- 

 rounded by what appears to be a clear space ; and at an earlier 

 stage a similar clear zone frequently also occurs around indi- 

 vidual granules, or groups of granules, prior to their complete 

 coalescence (PI. 22, figs. 6-7, x). This clear area may abut 

 directly on to the germinal vesicle, or it may be separated 

 from it by a thin layer of protoplasm (PI. 22, figs. 11, 13, 14). 

 At about this time also, vacuoles begin to appear in the body 

 of the yolk nucleus (PI. 22, figs. 11 and 13). 



The appearance of the yolk nucleus as seen in sections 

 depends, of course, upon the point through which the section 

 is taken (PI. 22, figs. 9-12). In figs. 9 and 10 the position of the 

 yolk nucleus is marked only by the clear area which usually 

 surrounds it; in fig. 11 the yolk nucleus shows a crescentic 

 cross section, and it is seen to contain numerous vacuoles ; 

 while in PI. 22, fig. 12, it has the appearance of a cap overlying 

 the nucleus. 



The yolk nucleus gradually passes from a crescentic to a 

 hemispherical form (PI. 22, fig. 14) ; and its growth being 

 proportionately more rapid than that of the ef^g as a whole, 

 this hemispherical form becomes still more marked in later 

 stages, so that in certain sections the yolk nucleus may even 

 appear as a complete ring encircling the nucleus (PI. 22, 

 fig. 15). The vacuoles increase in number, and frequently con- 

 tain crystalloid bodies at this stage (PI. 22, figs. 13, 14, cr.). 



The yolk nucleus next loses its originally homogeneous 

 appearance and shows signs of degeneration. This is evidenced 

 by the appearance of a peculiar reticulate structure, the sub- 

 stance between the meshes staining less deeply than the net- 

 work (PI. 22, figs. 15, 16). The yolk nucleus then loses its 

 regular outline (PI. 22, figs. 16, 17), and it finally breaks up 

 into more or less finely-divided, darkly-staining fragments, 

 surrounded each by a clear zone. The process of fragmenta- 



