W. G. Vanl^aiiie — Einhryologn of JEustylodias. 289 



developed dui-iiig a brief stage only, and they are doubtless often 

 obscured by the densely crowded and dark colored yolk spheres. 

 The centrosomes and centrospheres moreover greatly resemble in 

 size and staining qualities the yolk spheres with black centers which 

 I have already alluded to. 



In Planocera I have distinguished the sperm-centrosome Avith cer- 

 tainty only in eggs in the late stages of the fii-st polar spindle and 

 in the second polar spindle stages. Earlier than this it is probably 

 also visible, but until definite radiations develop I cannot certainly 

 identify it. When the centrosome can be recognized it is still sin- 

 gle (Fig. 88). It is surrounded by a distinct centrosphere and by 

 short radiations Avhich can be seen to be made up of microsomes 

 even close up to the centrosphere. Subsequently the centrosome 

 divides into two (Fig. 39) which separate, the centrosphere mean- 

 while elongating (Fig. 40) and eventually dividing, forming two 

 asters. From the position of the aster or asters, which is in nearly 

 every case somewhere on the line which the spermatozoon has prob- 

 ably travelled in penetrating to the position where we find it, they 

 have separated from the spermatozoon at a considerably earlier 

 stage. If this surmise is correct it may explain one specimen which 

 for a time seemed very puzzling, in which the polar spindle lies 

 between the spermatozoon and the sperm aster. But if the sperm- 

 centrosome separates from the nucleus before the polar spindle 

 reaches its peripheral position, it is not unlikely that this Avould 

 occasionally happen to move between the sperm-nucleus and its 

 aster. 



Van der Stricht found sperm-centrosomes and asters only in the 

 eggs where the sperm-nucleus lay near the vegetative pole or near 

 the center of the q^^^. I do not find that the position of the sper- 

 matozoon has anything to do with their jJi'esence or apparent 

 absence. I have not succeeded in learning anything of the subse- 

 quent history of these asters or centrosomes, nor demonstrated any 

 connection between them and the cleavage centrosomes. 



The First Cleavage Spindle. 



For a long time after the second polar body has separated the 

 granular remains of the inner aster of the polar spindle continue 

 to be visible. It has already been described as an area free from 

 yolk where the cytoplasmic microsomes, which stain blue with luTma- 

 toxylin, are particularly abundant and conspicuous (Figs. 19« and 



Trans. Conn. Acad., Vol. X. August, 1899. 



19 



