558 CELL DIVISION IN EGGS OF CREPIDULA. 



spindle fibers are dissolved and taken into the daughter nuclei along with other 

 achromatin (interfilar substance, archiplasm). During later stages of mitosis 

 much of the archiplasm (achromatin) flows from the astral radiations into the 

 central area of the aster and spindle, the daughter chromosomes absorb achro- 

 matin, thus becoming vesicular, and these chromosomal vesicles then fuse into 

 several karyomeres and finally into a single nuclear vesicle the wall of which is 

 composed of an outer achromatic membrane and an inner chromatic one, while 

 the chromatin within the membrane takes the form, at different stages, of vesicular 

 walls, reticulum, or granules. (Pp. 520, 542, 543.) 



18. The diffusion phenomena between nucleus and cell body may be inter- 

 rupted or modified by changes in the environment (increased temperature, 

 decreased oxygen tension, ether, diluted sea water, hypertonic sea water) and 

 the material of the astral radiations may be withdrawn into the central area of 

 the amphiaster or into the ectoplasmic layer, or isolated portions of it may be 

 scattered through the cell body. These altered conditions not only prevent 

 diffusion between different substances but they bring about a shrinkage or 

 segregation of these substances, (figs. 94, 96-99, 116, 117, 183, 189, 190). Dur- 

 ing 1-cell and 2-cell stages when astral radiations are extensive and much achro- 

 matin from the germinal vesicle is distributed through the cell, isolated portions 

 of this achromatin form cytasters (figs. 183, 189, 190, 201); these may contain 

 central granules but not true centrosomes; they fuse together but rarely if ever 

 divide, and they never form the poles of nuclear spindles; the size of amphiasters 

 and of nuclei is inversely proportional to the number and size of these cytasters 

 (figs. 171-176, 183-190, etc.), thus indicating that the substance of the cytasters 

 is achromatin withdrawn from the amphiasters and nuclei; if by the continued 

 action of hypertonic solutions cytasters are prevented from fusing with amphi- 

 asters and nuclei they form during interkinesis delicate achromatic membranes, 

 thus becoming achromatic vesicles, or "nuclei without chromatin," which may 

 be widely scattered through the cell (figs. 174, 184-191). If the daughter 

 chromosomes are prevented by hypertonic solutions from absorbing achromatin 

 they remain a densely chromatic mass which does not become reticular or gran- 

 ular, and if resting nuclei with their chromatin in the form of a loose reticulum are 

 treated with hypertonic solutions, the chromatic net at once contracts into a 

 dense central mass, suggestive of normal synezesis, which is surrounded by achro- 

 matin and achromatic membrane (figs. 178-192). When the typical swelling 

 of the chromosomes, by the absorption of achromatin, is prevented, a delicate 

 achromatic membrane may form around the achromatin of the spindle area, 

 within which the daughter chromosomes form a dense chromatic mass (figs. 178- 

 182) ; or the achromatic vesicle may form on one side, or some distance from the 

 chromatic mass (figs. 184-188). 



19. Cytasters do not, in Crepidula, contain true centrosomes nor take part 

 in nuclear division; on the other hand nuclear centrosomes and asters arise, 

 during cleavage at least, by the division of preexisting centrosomes. If cell 



