CELL DIVISION IN EGGS OF CREPIDULA. 



559 



division is suppressed while centrosomal division proceeds many centrosomes 

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

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

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

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

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

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

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



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

 temperature, ether, carbonic acid, diluted and concentrated sea water) may 

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

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

 scattered along the length of the spindle, in which case there is left a chromatic 

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

 between chromosomes at the poles of the spindle lead to the formation of separate 

 vesicles or karyomeres (figs. 120, 121, 124, 136, 145-150, 174, 175, 192-223). 

 Such conditions superficially resemble amitoses, but are true mitoses; there is no 

 entirely conclusive evidence that amitosis ever occurs in the origin of the sex 

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

 (Pp. 547-553.) 



XII. 



Catalogue of Experiments on Nuclear and Cell Division 

 in Crepidula. 



(Every number represents a separate microscopic slide, and in most instances a separate experiment. 

 The average number of eggs on each slide is not far from one thousand. All preparations were fixed, stained 

 and mounted in balsam, as described on p. 504. All slides are preserved as permanent preparations and 

 have been repeatedly consulted.) 



I. Abnokmalitces Found in Nature. 



In addition to occasional abnormalities found among eggs which were prevailingly normal, the following 

 layings were prevailingly abnormal. 



Number. 



Stage. 



Abnormal Conditions. 



Results. 



942 



943 

 944 



945 



1-cell to young 



veliger. 

 1-cell to gastrula. 

 1-cell to gastrula. 



24-cells to young 

 veliger. 



Water warmer and less 



dense than normal. 

 Unknown. 

 Unknown. 



Unknown. 



Ectodermal cap not overgrowing yolk; exogastrulae 



and exolarvse, figs. 27-29. 

 Giant eggs caused by fusion of two or more eggs. 

 Eggs vary in diameter from 96 y. to 192 p, probably 



due to fragmentation or fusion of eggs under 



pressure. 

 Ectodermal cap not overgrowing yolk; exogastrulae 



and exolarvse. 



