202 CORA J. BECKWITH 



E. DIFFERENTIATION OF THE EGG 



1. Nucleus 



The nuclear structure will be described in detail in a later sec- 

 tion, but may be considered briefly here. After certain killing 

 fluids (Flenuning, Meves, neutral formalin, hot water) the nucleus 

 early loses its affinity for all basic stains and shows a very fine, 

 nearly homogeneous structure, in which little or no evidence of a 

 fine net or chromosomes appears. It now takes even the plasma 

 stains very slightly (figs. 21, 28). After some killing fluids 

 (subhmate-acetic, picro-acetic, alcohol), a heavy net-like precipi- 

 tate, which takes basic stains strongly, is formed (fig. 22) . The 

 nucleus, which reaches a relatively enormous size, lies at first in 

 the center of the egg (fig. 19) and moves to the periphery only a 

 short time before growth of the egg is completed (fig. 23). A 

 basic staining nucleolus, which contains one or several vacuoles 

 staining with acid dyes, is constantly present. 



2. Protoplasmic changes 



a. Development of the simple yolk. The first elements to appear 

 in the protoplasm are small yolk-spheres which develop into the 

 the simple yolk and possibly the compound yolk-spheres of the 

 mature egg. It seems evident that these spheres develop directly 

 from the pseudochromatin-granules as described by Smallwood 

 ('09). Material fixed in Flemmings or Meves' fluid or formalin 

 is best for these observations, since the individual yolk-spheres 

 are kept distinct. In eggs about one-fourth grown, which have 

 been stained according to Benda's method, the extra-nuclear 

 granules can be seen to have enlarged slightly and uniformly 

 (fig. 9). They still take the yellow-brown color which Benda's 

 stain gives the granules. In a slightly older egg (about one-third 

 grown) some of the spheres are seen to have grown more rapidly 

 than the others, so that the uniformity in size is lost (fig. 10). 

 A few now increase in size very rapidly, giving yolk-spheres of 

 very unequal sizes (fig. 11). It is important to notice that the 

 spheres still stain like the pseudochromatin-granules (yellow- 



