MORPHOLOGICAL COMPOSITION OF THE NUCLEUS 299 



Boveri's hypothesis has been criticised by many writers, especially 

 by Hertwig, Guignard, and Brauer, and I myself have urged some 

 objections to it. Recently, however, it has received a support so 

 strong as to amount almost to a demonstration, through the remark- 

 able observations of Riickert, Hacker, Herla, and Zoja on the 

 independence of the paternal and maternal chromosomes. These 

 observations, already referred to at page 208, may be more fully re- 

 viewed at this point. Hacker ('92, 2) first showed that in Cyclops 

 strenuus, as in Ascaris and other forms, the germ-nuclei do not fuse, 

 but give rise to two separate groups of chromosomes that lie side by 

 side near the equator of the cleavage-spindle. In the two-cell stage 

 (of Cyclops tennicornis) each nucleus consists of two distinct though 

 closely united halves, which Hacker believed to be the derivatives of 

 the two respective germ-nuclei. The truth of this surmise was demon- 

 strated three years later by Ruckert ('95, 3) in a species of Cyclops, 

 likewise identified as C. strennns (Fig. 146). The number of chromo- 

 somes in each germ-nucleus is here twelve. Ruckert was able to 

 trace the paternal and maternal groups of daughter-chromosomes not 

 only into the respective halves of the daughter-nuclei of the two-cell 

 stage, but into later cleavage -stages. From the bilobed nuclei of the 

 two-cell stage arise, in each cell, a double spireme and a double 

 group of chromosomes, from which are formed bilobed or double 

 nuclei in the four-cell stage. This process is repeated at the next 

 cleavage, and the double character of the nuclei was in many cases 

 distinctly recognizable at a late stage when the germ-layers were 

 being formed. 



Finally Victor Herla's ('93) and Zoja's ('95, 2) remarkable obser- 

 vations on Ascaris showed that in Ascaris not only the chromatin of 

 the germ-i iclei, but also the paternal and maternal chromosomes, 

 remain perfectly distinct as far as the twelve-cell stage certainly a 

 brilliant confirmation of Boveri's conclusion. Just how far the dis- 

 tinction is maintained is still uncertain, but Hacker's and Riickert's 

 observations give some ground to believe that it may persist through- 

 out the entire life of the embryo. Both these observers have shown 

 that the chromosomes of the germinal vesicle appear in tivo distinct 

 groups, and Ruckert suggests that these may represent the paternal 

 and maternal elements that have remained distinct throughout the 

 entire cycle of development, even down to the formation of the egg ! 



Leaving aside all doubtful cases (such as the above suggestion of 

 Riackert's), the well-determined facts form an irresistible proof of the 

 general hypothesis ; and it is one with which every general analysis 

 of the cell has to reckon. I believe, however, that the hypothesis has 

 received an unfortunate name ; for, except in a few special cases, 1 



1 Cf. p. 273. 



