MORPHOI.OCICAL COMPOSiriON OF THE NUCLEUS 219 



prepares for division, however, the chromosomes contract, withdraw 

 their processes, and return to their '' resting state," in which iission 

 takes place. Applying this conclusion to the fertilization of the egg, 

 Boveri expressed his belief that "we may identify every chromatic 

 element arising from a resting nucleus with a definite element that 

 entered into the formation of that nucleus, from which the remark- 

 able conclusion follows tJiat in all cells derived in the regular course 

 of division from the fertilized egg, one-half of the chroviosonies are of 

 strictly paternal origin, the other Jialf of maternal ^ ^ 



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 re- 

 markable observations of Ruckert, Hacker, Herla, and Zoja on the 

 independence of the paternal and maternal chromosomes. These 

 observations, already referred to at p. 1 56, may be more fully reviewed 

 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 

 tenuicornis)&2iQh nucleus consists of two distinct though closely united 

 halves, which Hacker believed to be the derivatives of the two respec- 

 tive germ-nuclei. The truth of this surmise was demonstrated three 

 years later by Ruckert (95, 3) in a species of Cyclops, likewise identi- 

 fied as C strenuus {¥\g. 105). The number of chromosomes in each 

 germ-nucleus is here twelve. Ruckert was able to trace the pater- 

 nal 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 arises, 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 recog- 

 nizable at a late stage when the germ-layers were being formed. 



Finally Victor Herla' s remarkable observations on Ascaris ('93) 

 showed that in Ascaris not only the chromatin of the germ-nuclei, 

 but also the paternal and maternal chromosomes, remain perfectly 

 distinct as far as the twelve-cell stage — certainly a brilliant confirma- 

 tion of Boveri's conclusion. Just how^ far the distinction is main- 

 tained is still uncertain, but Hacker's and Riickert's observations 

 give some ground to believe that it may persist throughout the 

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



1 '91, p. 410. 



