MITOSIS AND AMITOSIS. 407 



shrinkage of the chromatin on the other hand probably indicates 

 that it is surrounded by a plasma membrane, or more likely 

 that each chromosome is so surrounded. 



When resting nuclei are subjected to hypotonic solutions the 

 entire nucleus becomes slightly swollen and less deeply chromatic, 

 which indicates that the chromatic parts of the nuclei take up 

 water, probably through the chromatic nuclear membrane or 

 the chromosomal membranes. 



The achromatic membrane, or rather boundary, is regularly 

 spherical in resting stages but during mitosis it disappears or 

 else becomes so indefinite and irregular in outline that it is 

 difficult to recognize. However the achromatic substance of 

 the nucleus together with some of the denser portion of the 

 cytoplasm constitutes the amphiaster with its nuclear spindle 

 and astral radiations. In hypertonic solutions the amphiaster 

 is sharply set off from the surrounding cytoplasm (Figs. 35-36), 

 due as I believe in the main to the condensation of its substance 

 and the elimination from it of the more fluid cytoplasm. In 

 this process of condensation the astral radiations are largely 

 drawn into the central part of the figure but portions of these 

 radiations may become isolated from the amphiaster and thus 

 form independent condensation centers. These have a radiating 

 structure and are typical cytasters, but unlike those described 

 by Wilson they do not in Crepidula divide nor form the poles of 

 true mitotic figures. My observations on the origin and nature 

 of these cytasters (Conklin, 1912) entirely agree with those of 

 Konopacki (1911) and in the main with the observations of 

 Mead (1898) and Morgan (1899). 



Cytasters appear best developed during periods of mitosis 

 when the achromatin is distributed in the astral radiations 

 (Figs. 33-35) but they are also abundant in eggs after the 

 maturation divisions and before the first cleavage (Figs. 31, 32) 

 and in such cases one can frequently see that they lie along the 

 radiations of the maturation aster (Fig. 31). 



During prolonged resting periods, especially when the eggs are 

 in strong salt solutions, cytasters are replaced by faintly staining 

 vesicles (Figs. 37-42) which appear to contain achromatic nuclear 

 material. These vesicles are surrounded by a delicate achromatic 



