CELL-ORGANS OF SEA-URCHIN EGG 575 



study of chromatin differentiation will be made in partheno- 

 genetic eggs first. 



Figure 7 shows a nucleus of a mature, unfertilized ovum under 

 high magnification. In all three sections, which contained parts 

 of the egg no chromatin granules whatever could be traced within 

 the nucleus. The highest magnification did not disclose in it 

 any definite structure except that it seemed to be finely reticular 

 or finely granular. Some of the nuclei of other unfertilized eggs 

 contain one or two intensely stained granules closely adjacent 

 to the membrane and even flattened upon it (fig. 2). Those 

 granules within the nucleus show the same staining reaction and 

 the same morphological appearance as the chromatic accumula- 

 tions in the cytoplasm. A well marked membrane is charac- 

 teristic of the nucleus of an unfertilized egg. Obst and Conklin 

 have described continuity in a resting nucleus between the cyto- 

 plasmic reticulum and the network of the nucleus. Unfertilized 

 eggs do not show connections between the nucleus and the cyto- 

 plasm, but connections between the cytoplasm and the nuclear 

 membrane may be easily traced in developing eggs. 



Figures 8, 9, 10, 11, 12 and 13 represent sections of partheno- 

 genetic eggs. An interesting feature, common to all these eggs, 

 is the formation of intensely staining particles immediately 

 within its nuclear membrane. This chromatic basophilic sub- 

 stance, which appears within the nucleus and which without 

 doubt contributes to the formation of chromosomes, may cer- 

 tainly be called chromatin. 



A comparison of the nucleus of an unfertilized egg on figure 7 

 with the nucleus of an egg in the beginning of parthenogenetic 

 development on figure 8 indicates merely slight changes around 

 the nucleus on the figure 8. The nuclei on both figures are nearly 

 equal in size; they both maintain a distinct membrane. The 

 only difference between them consists in accumulations of baso- 

 philic granules within and without the membrane of the nuclei. 

 The presence of the nuclear membrane allows to easily recognize 

 which of the chromatic granules lie within the nucleus and which 

 of them are in the cytoplasm. Many granules lie immediately 

 in contact with the nuclear membrane; but most interesting are 



JOURNAL OF MORPHOLOGY, VOL. 2?, NO. 3 



