KARYOKINESIS. 27 



sphere, the other from the sperm sphere, we should expect to find a centrosome in 

 connection with each germ nucleus and with no connecting central spindle between 

 them. This is just what occurs. In figs. 50—51 the two centrosomes are so placed 

 as to suggest that one is related to the egg nucleus and the other to the sperm 

 nucleus, and in figs. 48-53 there can be no doubt about this fact. In no egg ex- 

 amined is there a trace of a central spindle connecting the two centrosomes until 

 after the centrosomes are in their definitive positions and the nuclear membrane is 

 broken down at the poles of the spindle, figs. 54-55. Even though the centrosomes 

 may lie in their definitive positions at an early stage, a thing which sometimes 

 occurs (fig. 52), they are still quite independent, there being no central spindle fibres 

 between them. This evidence, therefore, although not entirely conclusive, is favor- 

 able to the view that one of the centrosomes of the first cleavage spindle comes from 

 the egg sphere and the other from the sperm sphere. 



Such a conclusion as to the origin of the cleavage centrosomes is at variance 

 with all observations which have been made heretofore, 1 and it is with much hesi- 

 tation that I bring it forward without being able to demonstrate its truth in the 

 clearest and most satisfactory manner. I have finally determined to publish these 

 observations only after having spent several years in trying to get indisputable 

 evidence upon this point, so far without success. However, the evidence, as far as 

 it goes, points to the conclusion that both egg and sperm spheres contribute to the 

 formation of the cleavage centrosomes. 



In view of the fact that, in Crepidula, egg and sperm centrosomes and spheres 

 undergo parallel metamorphoses and that both spheres persist until their union, the 

 commonly accepted view that the spermatozoon alone contributes to the cleavage 

 centrosomes seems in this case highly improbable. Further, there is no particle of 

 direct evidence in favor of this view ; there is no sperm amphiaster as in many 

 other cases ; when the cleavage centrosomes first appear there is no central spindle 

 between them, as would be the case if both were derived from a single sperm cen- 

 trosome ; a centrosome usually appears in connection with each germ nucleus, which 

 is also inexplicable on the supposition that both have come from the spermatozoon. 

 These same facts are equally strong against the supposition that both cleavage cen- 

 trosomes are derived from the egg centrosome. 



On the other hand it is quite possible that both cleavage centrosomes are new 

 formations, i. e., are not directly derived from the egg and sperm centrosomes, but 

 have arisen independently of these and of each other, in the remains of the fused 

 spheres. Apart from the evidence that one centrosome comes from each of the 



1 It most closely resembles the results of Carnoy and Lebrun ('97) on Ascaris, though it differs 

 fundamentally from these in that these authors claim that the cleavage centrosomes arise from nucleoli, 

 one of which comes from each of the germ nuclei. 



Since the above was written Lillie's (1901) complete paper on the maturation, fertilization and 

 cleavage of Unio has appeared, and the account which he gives of the origin of the cleavage centro- 

 somes in that animal is strikingly like my observations as to the origin of these centrosomes in Crepi- 

 dula. In brief he finds that one cleavage centrosome arises in connection with each germ nucleus, that 

 there is no central spindle between them and that they arise near or in the margin of the sphere sub- 

 stance. He does not consider that they are descendants of the egg centrosomes or sperm amphiaster, 

 but that they are egg products of new origin. 



