The maturation and fertilization of the egg of Cerebratulus. 455 



asters may arise far apart and later come together to form the spindle 

 by "the fusion of certain groups of their rays". MacFarland believes 

 he is justified in the conclusion that "the centrosomes which form the 

 poles of the first cleavage-spindle arise exclusively from the spermato- 

 zoon ; they have retained their independence throughout the whole 

 time, although they were not continually visible during the whole 

 course of fertilization". 



Klinckowström's account of the process of fertilization in the 

 Planarian Prostheceraeus (12) is essentially similar. He finds that 

 "sowohl bei dem männlichen als bei dem weiblichen Vorkern werden 

 jedoch im Laufe der Entwicklung die Strahlungen immer undeutlicher, 

 bis sie bei dem ruhenden Vorkern vollständig verschwunden sind. 

 Gleichzeitig mit dem Wiederauftreten der Kernsegmente werden auch 

 zwei Centralkörperchen sichtbar. Die beiden Centralkörperchen , die 

 von rasch wachsenden Polstrahlungen umgeben sind, liegen ziemlich 

 weit von einander entfernt der Kernmembran dicht an; eine Ceutral- 

 spindel scheint vollständig zu fehlen" ^). The observations of both of 

 these authors agree perfectly with those of similar stages which 

 Mark (19) has described for Limax ^ with Boveri's description (3) 

 of Pterotrachea^ etc., and with others. They differ from the account 

 given above for Cerebratulus chiefly in the much earlier disappearance 

 of the sperm-asters. 



In the Echinoderm it is well known that the fibres of the sperm- 

 asters greatly diminish in number, size and length at the time of the 

 union of the germ-nuclei. This is well shown in Wilson's admirable 

 account of the process in Toxopneustes (34). Observations which I 

 have made on the eggs of Echinus^ of Sphaerechinus, and of Strongylo- 

 centrotus lead me to believe that this diminution of the asters just 

 before the union of the germ-nuclei is much greater than has been 

 described. In Thalassema (7), in Toxopneustes (34), and in many 

 other eggs the asters become far less distinct at this stage. 



It would appear, therefore, that the eggs of these and many 

 other animals may be arranged in a series which will show the 

 successive gradations in the disappearance of the sperm-asters. At 

 the head of the series we must place the eggs of Ascaris, of Myzo- 

 stoma (according to Kostanecki) and similar ones in which the 

 sperm-asters make their appearance only a short time before the 

 formation of the cleavage-spindle, and which, consequently, suffer no 



1) p. 600. 



Zfol. Jahrb. XII. Abth. f. Morph. gQ 



