CELL DIVISION IN EGGS OF CREPIDILA. 



559 



division is suppressed while centrosomal division proceeds many ntroaom 

 and asters (poly asters) appear in the cell, but there is no evidence that these 

 arise de novo or from cytasters. If the union of germ nuclei, after fertilimtioii 

 is delayed, the egg centrosome may divide, giving rise to a spindl and at the 

 same time a spindle may form in connection with the sperm nucleus; if then 

 spindles are far apart they remain independent, if near together they interfere, 

 thus producing tetrasters or triasters (figs. 159-170). The cleavage ceatr 

 do not invariably arise from the sperm centrosome. (Pp. 542-547.) 



20. Many different environmental changes (shaking, praeure, increased 

 temperature, ether, carbonic acid, diluted and concentrated set water) ma 

 cause abnormalities in the separation of daughter chromosomes, and in thai 

 fusion to form daughter nuclei. Chromosomes may lag in the equator or be 

 scattered along the length of the spindle, in which case then- is 1. ft a rhromaflfo 

 connection between daughter cells (figs. 140-144, 152-154); slight separation* 

 between chromosomes at the poles of the spindle lead to the format ion of separate 

 vesicles or karyomeres (figs. 120, 121, 124, 136, 145-150, 174, 175, 192-228). 

 Such conditions superficially resemble amitoses, but are true mitoses; there Is ao 

 entirely conclusive evidence that amitosis ever occurs in the origin of tl ex 

 cells of metazoa or in the divisions which accompany embryonie differentiation. 



(Pp. 547-553.) 



r 



XII. 



Catalogue of Experiments on Nuclear and Cell Divisk 



in Crepidula. 



(Every number represents a separate microscopic slide, and in most instano- ncpirat* expcnmMt 



The average number of eggs on each slide is not far from 



preparation 



fixed. ita>n«i 



mounted in balsam, as described 



All slides are 



I •«• 



prep 



and 



have been repeatedly consulted 



I. Abnormalities Found in Nature, 



In addition to occasional abnormalities found among eggs which were prevailing normal, the Mowing 



layings were prevailingly abnormal. 



Number, 



Stage. 



Abnormal 



Rem 



942 



943 

 944 



1-cell to young 



velige 



i V 



gastrula 



945 



1-cell to gastrula 



24-cells to young 

 veliger. 



Water warmer and less' Ectodermal cap not overgrowing yolk; exo|*rul* 



and exolarvae, figs. 27-29. 



Giant eggs caused by fusion of two or more <W* 

 Eera vary in diameter from 96* to 1<J2* P r **£ 

 due to fragmentation or toon of *p *"*» 



dense than normal 

 Unknown. 

 Unknown. 



Unknown 



Ectodermal cap not overpwin* yolk e»*i*ml* 



and exolarvw. 



