208 FERTILIZATION OF THE OVUM 



in this direction, was unable to follow the paternal and maternal 

 chromatin beyond the first cleavage-nucleus, though he surmised that 

 they remained distinct in later stages as well ; but 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. 146). 

 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 univalens having one 

 chromosome, the three chromosomes reappear at each cleavage, at 

 least as far as the twelve-cell stage (Fig. 145); 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. THE CENTROSOME IN FERTILIZATION 



In examining more critically the history of the centrosomes we may 

 conveniently take Boveri's hypothesis of fertilization as a point of 

 departure, since it has long formed the focus of discussion of the 

 entire subject. Before the hypothesis is more closely scrutinized we 

 may first eliminate two other views, both of which are irreconcilable 

 with it, though neither has stood the test of later research. The first 

 of these, doubtfully suggested by Van Beneden ('87) and definitely 

 maintained by Wheeler ('97) in the case of Myzostoma, is that the 

 cleavage-centrosomes have no definite relation to the spermatozoon, 

 but are derived from the egg a conclusion that has the a priori 

 support of the fact that in parthenogenesis the centrosomes are cer 

 tainly of maternal origin. 



Van Beneden's early statement may be passed by, since it was no 

 more than a surmise. Wheeler, after a careful research, found that no 

 sperm-aster accompanied the sperm-nucleus a fact correlated with 

 the absence of a middle-piece in the spermatozoon, and reached the 

 conclusion that after formation of the polar bodies, the egg-centro- 

 somes persisted to become directly converted into the cleavage-centro- 

 somes (Fig. 104). That the absence of a distinct middle-piece is not a 

 valid argument is shown by the insect-spermatozoon, where the region 



