242 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



ently active. When we come to the stage represented in their Figure 

 33a, and still further to conditions such as [ have found where no aster 

 can be detected in connection with either egg-nucleus or sperm-nucleus, 

 then we may well question the value of the " assimilation " theory. 



I have referred in an earlier part of this paper to the clear evideuce 

 brought forward by Hill ('95) to show that in the egg of Sphserechinus 

 the egg-aster completely disappears. The sperm-aster, from the time it 

 appears at the base of the sperm-head till the first cleavage spindle is 

 formed, is clearly visible ; and the sperm-centrosome and its product, 

 the sperm-spindle, are accompanied by the only astral structures that 

 are visible. Fick ('93) finds the first cleavage spindle to be of spermatic 

 origin, but the proof to be found in the existence of an " archoplasmic " 

 mass is not so satisfactory as the evidence produced by Hill. 



Some of the most recent work on the history of the centrosomes in 

 the egg tends to re-open the question of the origin of the first cleavage 

 spindle. Foot ('97) maintains for Allolobophora that whatever the evi- 

 deuce of an aster about the spermatozoon .in the egg, the cleavage aster 

 comes from the egg itself. According to this investigator, the sperma- 

 tozoon gives to the developing embryo only the sperm-head. The first 

 cleavage spindle comes fi'om the egg-aster, although that structure is 

 invisible during the resting stage of the egg-nucleus. Klinckowstrom 

 ('97) finds in a planarian that both egg-aster and sperm-aster disappear 

 during the resting stage of the germ-nuclei. When two asters make 

 their appearance to form the first cleavage spindle, they are at a con- 

 siderable distance from each other and in such relation to the two germ- 

 nuclei that it is impossible to decide their origin. 



The conditions encountered by Klinckowstrom are apparently the 

 same as those represented in my Figures 12, 18, and 19, referred to above ; 

 and since he, as I, could not follow the changes in the centrosome con- 

 tinuously, we can not say, as others have said of centrosomes in practi- 

 cally the same relation to nuclei, that both are of spermatic origin. 

 Hence, if anything leading toward conclusive proof is to be known, it 

 must be learned from facts striking enough to counterbalance the weak- 

 ness incident to a break in the series of phases through which the cen- 

 trosomes are seen to pass. I believe I have in Plate 4, Figures 26, 27, 

 and 28, evidence which proves, just at the stage of development when 

 evidence is most conclusive, that the first cleavage spindle is wholly of 

 spermatic origin. 



The relation of egg-nucleus and sperm-nucleus in the egg of Limnaea 

 is so constant that the sperm-nucleus is never found between the polar 



