574 A. G. WALTON 



through all the stages of metamorphosis into the mature sperma- 

 tozoon; secondly, afterwards during the entrance of the sperm 

 into the egg, and then, thirdly, as the centrosome which appears 

 after the entrance of the sperm and functions at cleavage. While 

 the kinetic continuity of the male centrosome has long been known 

 to exist, evidences of its morphological continuity have been very 

 scanty. We find that the centrosome becomes imbedded in the 

 side of the nucleus of the metamorphosing spermatid, — perhaps 

 in a similar way to the imbedding of the anterior centrosome in 

 the nucleus of a mammalian sperm, — and is carried into the egg 

 by the sperm. Here it divides, and its halves, still connected 

 by a 'central-desmus', migrate out from the sperm nucleus as 

 the latter forms into discrete chromosomes. Taking up a posi- 

 tion in the egg cytoplasm at some distance from the sperm, it 

 becomes surrounded by a faintly distinguishable aster, the origin 

 of which — whether from the sperm or the egg cytoplasm — is as 

 yet not definitely determined. From careful observation, how- 

 ever, it seems most probable that the immediately surrounding 

 cytoplasm of the aster is mostly of male origin and the more 

 peripheral portions mainly female. There is no definite line of 

 demarcation, since the transition from the cytoplasm of one source 

 to that of the other is very gradual. The later separation of the 

 sister centrosomes, concomitant with the approach of the two 

 pronuclei towards the center of the egg, and the final setting up 

 of the cleavage spindle with the two centrosomes, both of male 

 origin, is entirely normal in character. 



The entrance and subsequent metamorphosis of the sperm 

 before the giving off of the first polar cell may well give rise to 

 questions as to the time and manner in which fertilization is 

 accomplished. If we adhere to the definition of fertilization as 

 the intermingling of the chromatic material of the egg and sperm, 

 such an action can take place only after the formation of the resting 

 nuclei of the first two cleavage cells. It is here, and here only, 

 that the chromatic material of the two gametes has a chance to 

 intermingle and that the process of synmixis takes place. This 

 definition is advocated by Zacharias ('12) and by Vejdovsk>^ 

 ('12). The latter has furnished good arguments for his view, 



