STUDIES ON CELL DIVISION AND CELL DIFFEREN- 

 TIATION 



I. DEVELOPMENT OF THE CELL ORGANS DURING THE FIRST CLEAV- 

 AGE OF THE SEA-URCHIN EGG 



VERA DANCHAKOFF 



Thirty-one figures (five plates) 

 1. INTRODUCTION 



The morphological changes of the living substance, exhibited 

 in our preparations, refer only to certain stages of the complex 

 physico-chemical process which is involved in the history of the 

 cell. Unfortunately, isolated phases of cell development often 

 become the subject of independent studies and the inseverable 

 connection between the different stages of cell development 

 became gradually lost. 



The paricular interest attaching to the nucleus began with its 

 discovery. Observations on its physiological activity seemed to 

 indicate it as the regulative center of the metabolic processes in 

 the cell. The discovery of chromatin in the nucleus and of the 

 differentiation of chromosomes during cell division attracted 

 the attention of investigators to the specific role of the nucleus 

 as bearer of hereditary qualities. And both functions relating 

 to the nucleus, different as they may be, seemed to find a material 

 basis in the chromatin, which was admitted to be localized not 

 only within the nucleus, but also in the cytoplasm. 



Investigators have repeatedly described the presence of 

 chromatin in the cytoplasm of both protozoa and metazoa. 

 Although the nucleus was invariably considered as the permanent 

 primary source of the extranuclear chromatin, the interpreta- 

 tion of the role assigned to the extranuclear chromatin has widely 

 differed. Hert wig's followers assume a permanent presence of 

 chromatin in the cytoplasm. Hertwig himself gave to the extra- 



559 



JOURNAL OF MORPHOLOGY, VOL. 27, NO. 3 

 SEPTEMBER, 1916 



