68 



FUNDAMENTALS OF CYTOLOGY 



their prophasic changes and the nuclear membrane disappears, all the 

 elements concerned — amphiaster, karyolymph, chromosomes — establish- 

 ing the metaphase figure. In many cases each centriole is now clearly 

 double, ha\'ing divided nearly one nuclear cycle in advance of the i^ro- 



FiG. 48. — Diagram of astral mitosis and cytokinesis in an animal cell. 



phase in which its halves are to separate. Such astral figures are also 

 found in certain fungi and algae (Fig. 49). 



The asters, in the possession of which the mitotic figure differs so 

 conspicuously from that in higher plants, are evidently developed by 

 the formation and gradual extension of centripetally moving streams, 

 or "astral rays," in the cytoplasm about the centrosomes. The regions 



X 







It 



''V/v^^T-^,. ;h 







Fig. 49. — Astral mitosis in brown algae, a, centrioles with asters moving apart along 

 nuclear membrane in apical cell of Stypocauloii. b, metaphase in oogonium of Fucus; the 

 spindle is intranuclear, (a, after W . T. Swingle; b, after S. Yamanouchi.) 



between the streams are gelled, so that the whole aster, in spite of its 

 fluid streams, has a relatively firm consistency and can be moved about 

 in the more fluid cytoplasm with a micro-needle. When one of the 

 minute oil-like droplets occasionally seen in the sand-dollar egg is pushed 

 from the fluid portion of the cytoplasm into the periphery of the aster, 



