580 VERA DANCHAKOFF 



matic male pronucleus, and both pronuclei unite in very similar 

 condition — an achromatic segmentation nucleus resulting from 

 this union (fig. 16). Finally the male pronucleus may have dif- 

 ferentiated its chromosomes before union proceeds and during 

 copulation it adds to the female pronucleus already differen- 

 tiated chromosomes (fig. 17). 



The differentiation of the chromosomes is easier to follow in 

 cases, where the nuclei join together in an achromatic stage. It 

 proceeds then similarly to what is seen during parthenogenesis (fig. 

 16). The membrane of the segmentation nucleus is thin if it 

 exists at all. The first appearance of the chromatic particles 

 occurs invariably in the immediate vicinity of the membrane; 

 they appear in section as lines, grow very rapidly to spherical 

 bodies, flattened against the membrane, with marked convexity 

 toward the center of the nucleus. Figure 16 shows the forma- 

 tion of larger bodies which remind the nucleoli. On the cyto- 

 plasmic side these nucleoli are surrounded by a denser cytoplasmic 

 net. 



The conditions within and without the nucleus must be dif- 

 ferent at this time, for the chromatic substance, once it has 

 entered the nucleus, does not again leave it. The nucleolus con- 

 tinues to grow by the side next the cytosome. At the same 

 time its substance becomes loose on the opposite side. This 

 loosening of the nucleolus is quite similar to that of the sper- 

 matozoon, when it joins the female pronucleus in a dense state. 

 A comparison of figures 14 and 16 will show, that at the peri- 

 phery of the nucleolus as well as at the periphery of the spermato- 

 zoon, numerous threads appear, which grow toward the center 

 of the nucleus. 



The figures 14, 15 and 17 illustrate the resorption of chromatic 

 substance by the segmentation nucleus in cases, when the male 

 pronuclei add to the female pronuclei considerable quantities of 

 chromatin, either in an unchanged form (fig. 14), or in the form 

 of already differentiated chromosomes (fig. 17). The resorption 

 of the cytoplasmic basophilic substance proceeds partly in the 

 form of numerous small spots on the membrane, partly in the 

 form of one or of two well differentiated nucleoli. 



