RAND: NERVOUS SYSTEM OF LUMBRICID^. 153 



cell membrane, as well as the newly formed equatorial membrane, de- 

 generates and disappears, leaving the two daughter chromatic masses, 

 connected by the equatorially constricted interzonal filaments, lying 

 free in an indefinitely outlined clear space. 



c. The daughter chromosomes, having completed their migration 

 toward the poles, fuse to form solid chromatic masses, concave on their 

 polar sides. The chromatic mass represents the nucleus. 



d. In the concavity of each chromatic mass is a sharply outlined, 

 lightly stained, conical region, — the region of the old spindle-end, — 

 with the centrosome at its apex. This conical polar region is the funda- 

 ment of the cytoplasm of the young cell. 



e. The interzonal filaments disappear, leaving the two chromatic 

 masses and their accompanying polar structures lying free in a com- 

 mon, irregularly defined, clear space formerly occupied by the mother 

 cell. All- the living parts of each daughter cell are comprised within the 

 chromatic mass and its polar region. 



f. The two young sister cells may become separated by the pushing 

 in of adjacent tissue between them. 



g. The transformation of the chromatic mass into a resting nucleus 

 involves the swelling of the newly formed nuclear membrane, the in- 

 crease in the volume of its contents probably being due to the absorp- 

 tion of fluid material. The solid ma.ss of chromatin meanwhile gradually 

 breaks up into small granules which, together with the achromatic sub- 

 stance, form the loose peripheral nuclear network of the resting nucleus. 

 During this process a nucleolus appears. 



h. Accompanying these changes in the nucleus, the polar cytoplasmic 

 fundament increases in volume, remaining always sharply outlined in 

 the surrounding clear space. For a considerable period the growing 

 cytoplasm is massed upon the concave polar side of the nucleus. At 

 length the nuclear concavity disappears and the increasing cytoplasm 

 envelops the entire nucleus. By far the greater mass of cytoplasm, 

 however, always remains on the polar side of the nucleus. 



i. The size and form of the typical pear-shaped nerve cell are finally 

 attained as a result of the continued swelling of the nucleus, the increase 

 in volume of its large polar body of cytoplasm, and the development of 

 a nerve process from the smaller mass of cytoplasm on the equatorial 

 side of the nucleus. 



j. Distinct centrosomes are to be seen at the poles of the mitotic 

 figure at all stages. 



After the disappearance of the interzonal filaments the centrosome is 



