28 KARYOKINESIS. 



spheres, there is no reason to be alleged why both may not be new formations with- 

 out genetic relationship to egg or sperm centrosomes, except the analogy of the 

 cleavage stages, where a persistence of centrosomes in all stages can be clearly 

 established. 



In a former account of the fertilization of Crepidula (Conklin '94) I described 

 a form of " Quadrille of the Centers," in some respects similar to that observed by 

 Fol ('91) and Guignard ('91). In this account I expressly stated that I had not 

 seen the centrosomes during the fertilization, but only the egg and sperm " asters." 

 My account of the persistence and approach of both the asters until they come into 

 contact, I am now able to confirm. However, my account of their subsequent divi- 

 sion into halves and the union of these halves by pairs to form the cleavage asters 

 was incorrect. Judging by what I have since seen I am convinced that in my 

 former paper I mistook lobulations of the egg and sperm spheres such as are shown 

 in fig. 44, for division of those spheres, and other similar lobulations of the fused 

 spheres, figs. 46-49, for the union of half-spheres to form the cleavage asters. 



The present stand of the question of the centrosomes in fertilization is so well 

 known that it demands no extensive treatment here. Following the publications 

 of Fol, Guignard, Blanc and myself, papers on this subject came "fast and furious." 

 Boveri ('95), Wilson and Mathews ('95), Hill ('95) and Reinke ('95), showed that 

 no quadrille occurred in the Echinoderms ; Kostanecki and Wierzejski ('96), Mac- 

 Farland ('97), and more recently Griffin ('99), and Linville (1900), held that it did 

 not occur among mollusks; Guignard' s work has failed of confirmation by other 

 writers; Van der Stricht ('95), who held that a quadrille occurred in Amphioxus, 

 has been followed by Sobotta ('97), who maintains that there is no quadrille in that 

 animal, and Blanc ('93), who described a form of quadrille in the trout, has been 

 followed by Behrens ('98), who finds that both the cleavage centrosomes in that 

 animal comes from the sperm ; and so the quadrille went to its death. 



On the other hand Boveri's ('87-92) view that the cleavage centrosomes were 

 introduced by the spermatozoon, and that "zV is the centrosome alone which incites 

 the division of the egg, and is, therefore, the fertilizing element proper''' (Wilson 

 '96, p. 140), was eagerly championed by more than a score of writers; in fact this 

 doctrine was much more cautiously held by Boveri than by many of his followers. 

 However, there has been accumulating a body of evidence to show that the cleavage 

 centrosomes do not, in all cases at least, come from the spermatozoon. Apart from 

 the long known fact that cleavage centrosomes are present in parthenogenetic eggs, 

 many observations have been made on fertilized eggs which tend to show that these 

 centrosomes may come from the egg centrosome or may possibly arise independently 

 of either the egg or sperm centrosomes. I refer particularly to the work of Wheeler, 

 Foot, Mead, Lillie, Child and myself. In almost every case so far observed there 

 is a period, more or less prolonged, during which no centrosomes are visible (cf. Coe, 

 '98, p. 455). In only a few cases is it affirmed that the sperm centrosomes can be 

 traced without a break into the cleavage centrosomes. 



So far as the mollusks are concerned there does not seem to be a single case in 



