156 FERTILIZATION OF THE OVUM 



nal chromatin, but also the chromosomes, may retain their individu- 

 ality throughout development. 1 Van Beneden, the pioneer observer in 

 this direction, was unable to follow the paternal and maternal chro- 

 matin beyond the first cleavage-nucleus, though he surmised that they 

 remained distinct in later stages as well ; and Rabl and Boveri 

 brought forward evidence that the chromosomes did not lose their 

 identity, even in the resting nucleus. Riickert ('95, 3) and Hacker 

 ('95, i) have recently shown that in Cyclops, the paternal and mater- 

 nal chromatin-groups not only remain distinctly separated during the 

 anaphase, but give rise to double nuclei in the two-cell stage (Fig. 105). 

 Each half again gives rise to a separate group of chromosomes at 

 the second cleavage, and this is repeated at least as far as the bias- 

 tula stage. Herla and Zoja have shown furthermore that if in 

 Ascaris the egg of variety bivalens, having two chromosomes, be 

 fertilized with the spermatozoon of variety univalcns having one 

 chromosome, the three chromosomes reappear at each cleavage, at 

 least as far as the twelve-cell stage (Fig. 106); and according to Zoja, 

 the paternal chromosome is distinguishable from the two maternal at 

 each step by its smaller size. We have thus what must be reckoned 

 as more than a possibility, that every cell in the body of the child may 

 receive from each parent not only half of its chromatin substance, 

 but one-half of its chromosomes, as distinct and individual descendants 

 of those of the parents. 



C. CENTROSOME AND ARCHOPLASM IN FERTILIZATION 



We have now finally to consider more critically the history of the 

 centrosomes in fertilization, already briefly reviewed at p. 135. The 

 account there given considers only the more usual and typical history 

 of the centrosome, viz. the degeneration of the egg-centrosome and 

 the introduction of a new centrosome by the spermatozoon. There is, 

 however, one phenomenon which indicates a priori the possibility that 

 other modes of fertilization may occur, namely, parthenogenesis, in 

 which the egg develops without fertilization. In this case, as Brauer 

 ('93) has clearly shown in Artemia, the egg-centrosome remaining 

 after the formation of the polar bodies does not degenerate, but divides 

 into two to form the cleavage-amphiaster. The degeneration of the 

 egg-centrosome is therefore not a necessary or invariable phenome- 

 non, and as a matter of fact several accounts have been given of its 

 persistence and active participation in the process of fertilization. 

 These accounts fall under three categories, as follows : 



i. Each germ-cell contributes a single centrosome, one of which 



1 < T. p. 219. 



