CELL-ORGANS OF SEA-URCHIN EGG 589 



attraction by the two nuclei works in opposite directions, and a 

 division consequently takes place in this zone. 



The subsequent development of each of the daughter cells is 

 similar to the development of the mother cell, as are similar the 

 starting points of their development. Each blastomere (fig. 31) 

 has an achromatic nucleus, surrounded by more intensely stain- 

 ing cytoplasm; each blastomere contains numerous small baso- 

 philic granules in the cytoplasm. Several small chromatin par- 

 ticles appear at the inner side of the membrane; they grow and 

 form a chromatin net, at the expense of which the chromosomes 

 are differentiated. A concentration of the archoplasm (plasto- 

 somes) at the poles of the nuclei is effected through bipolar 

 radiations and the ingrowth of this substance into the nuclear 

 cavity contributes to the formation of the spindle. A study of 

 the first segmentation in the sea urchin egg does not contribute 

 any evidence for the assumption of a primary role of the centro- 

 some, nor for its self-propagation. The appearance of the cen- 

 trosome reveals itself as an important stage in mitotic process, 

 but like the appearance and disappearance of every other cell 

 organ, is closely and necessarily connected with certain physico- 

 chemical changes in the egg substance. 



7. SUMMARY 



1. Accumulations of basophilic chromatic substance are found 

 in the cytoplasm of mature sea-urchin eggs (Strongilocentrotus 

 lividus). 



2. After fertilization and at the beginning of artificial parthe- 

 nogenesis these basophilic accumulations undergo a dissolution. 

 This process is effected in parthenogenetic eggs more vigorously 

 and incites the appearance of cytasters. 



3. The small achromatic nucleus of the mature egg both after 

 fertilization and at the beginning of parthenogenesis seems to 

 exert a strong attraction on the basophilic accumulations found 

 in the cytoplasm. This substance is soon displaced and local- 

 ized around thenucleus. At the same time a radiation around 

 the nucleus is perceptible, which apparently is the expression 

 of the flowing of the basophilic substance toward the nucleus. 



