238 W. R. Coe — Development of the 



of the maturation spindle appear on that side of the eccentrically- 

 placed germinal vesicle which lies nearest the surface of the Qg^ ; 

 the polar bodies are formed on the same side ; the spermatozoon 

 usually enters at the opposite pole ; and the first cleavage plane 

 passes through the region of the polar bodies. 



After the entrance of the spermatozoon its head thickens up, and 

 a delicate aster with distinct centrosome appears in its immediate 

 vicinity. The centrosome, and later the aster, divides into two, with 

 the formation of a delicate spindle. The spindle is soon ruptured 

 and the two resulting asters may remain together near the si^erm- 

 head ; may separate widely from it and from each other ; or one of 

 them may wander off to a considerable distance, while the other 

 remains in the immediate vicinity of the sperm-nucleus. 



The centrosome remaining in the egg disappears after the forma- 

 tion of the second polar body, as do both of the sperm-asters some- 

 what later. The egg is thus left without visible centrosome, as 

 commonly occurs in eggs of other animals. In this respect they agree 

 most closely with Kostanecki and Wierzejski's account of the mollusk 

 Physa (18), and with Child's description (7) of the process in the 

 annelid Arenicola. The radiations of the sperm-asters, however, 

 remain long after the disappearance of the centrosomes, and in most 

 cases may be recognized even after the formation of the cleavage- 

 asters. 



From the evidence obtained from a few eggs in which the sperm- 

 centrosomes did not disappear as soon as usual, I am of the opinion 

 that these centrosomes do not actually end their existence with the 

 disappearance of the sperm-asters, and that they are identical with 

 those which later appear in the pair of cleavage-asters. 



The centrosomes of the cleavage-asters are surrounded by distinct 

 centrospheres which increase enormously in size with a very slight 

 increase in the size of the centrosomes. These centrospheres are not 

 artefacts, for they may be seen in the living egg. The centrosomes 

 divide very early. In the anaphase they separate somewhat, and 

 about each a delicate aster is formed quite within the body of the 

 centrosphere. These little asters extend their rays outward into the 

 reticulum of the egg and eventually form the asters of the second 

 cleavage, somewhat as Griffin has described in Thalassema (13). 

 Here, as in the later stages, there is certain proof of the existence 

 of the centrosome from one cell-generation to another. Its persist- 

 ence does not necessarily indicate, however, that it has any right to 

 claim for itself a place among the essential and permanent organs of 

 the cell. 



