582 VERA DANCHAKOFF 



6. DIFFERENTIATION OF CHROMOSOMES, OF THE SPINDLE AND OF 



THE CENTROSOME 



The study of parthenogenetic development in the sea urchin 

 eggs has been of great help in the study of chromatin differen- 

 tiation in the nucleus. The further development of partheno- 

 genetic eggs, the differentiation of chromosomes, of the spindle 

 and of the centrosome, becomes often very irregular. The anom- 

 alies observed in transportation and resorption of the chromatic 

 cytoplasmic substance influence the further development of the 

 egg and result in complex changes, which concern at the same time 

 different cell structures. Parthenogenetic eggs exhibit during 

 the first cleavage numerous and various anomalies in the devel- 

 opment of the mitotic figure, while in normal development the 

 corresponding structures show a perfect regularity. The further 

 observations on the cell organs will therefore be based principally 

 on the study of normally fertilized eggs. 



The development of chromosomes is closely connected with 

 that of the spindle and of the centrosome. According to dif- 

 ferences observed in the resorption of the basophilic substance 

 and in the copulation of the pronuclei, the development of the 

 chromosomes often exhibits a great diversity. The individual 

 variations in the development of the chromosomes are so numer- 

 ous, that their description would require a separate study. A 

 few examples are sufficient for my purpose : 



If the spermatozoon joins the female pronucleus in a dense 

 condition, which seems to be typical, its chromatin becomes 

 gradually looser within the segmentation nucleus. A meshwork 

 of chromatin threads results, which soon unites with chromatin 

 particles of the female pronucleus. Figure 15 illustrates similar 

 connections between the paternal and maternal chromatin. A 

 common network is gradually formed, from which the chromo- 

 somes are differentiated. 



Should the spermatozoon have changed its dense structure and 

 have become achromatic before the fusion of the pronuclei took 

 place, the segmentation nucleus in such cases appears almost 

 free of chromatin (fig. 16). The development of chromatin with- 

 in the achromatic segmentation nucleus will take place after 



