THE INDIVIDUALITY OF THE GERM-NUCLEI 



DURING THE CLEAVAGE OF THE EGG OF 



CRYPTOBRANCHUS ALLEGHENIENSIS. 



BERTRAM G. SMITH. 



Early observers of the process of fertilization described the 

 meeting of the sperm-nucleus and the egg-nucleus, and their 

 complete fusion to form a single zygotic nucleus, the primary 

 nucleus of the embryo. Later it was found that in many cases, 

 though apparently not in all, the two germ-nuclei merely become 

 apposed without fusion. In tracing their further history it was 

 shown, in certain cases, that the germ-nuclei give rise to two 

 independent groups of chromosomes which separately enter the 

 equatorial plate and whose descendants pass separately into 

 the daughter-nuclei. "Later observations have given the 

 strongest reasons for believing that, as far as the chromatin is 

 concerned, a true fusion of the nuclei never takes place 'during 

 fertilization, and that paternal and maternal chromatin may 

 remain separate and distinct in the later stages of development 

 possibly throughout life" (Wilson, 1900, p. 204). At the present 

 time there is much to warrant the belief that a fusion of maternal 

 and paternal chromatin never takes place in the somatic cells, 

 while in the lineage of germ cells nothing approaching fusion 

 occurs until a pairing of maternal and paternal chromosomes, 

 called synapsis, takes place in preparation for the maturation 

 divisions; then for the first time maternal and paternal chromo- 

 somes are brought together in intimate and orderly union, in 

 some cases amounting to fusion. 



This conclusion, which is of the most fundamental importance 

 since it vitally concerns the mechanism of heredity, is based 

 largely on indirect evidence; for in almost every case apparently 

 insuperable obstacles have been encountered in the attempt to 

 trace the respective maternal and paternal chromatin-complexes 

 through the resting stage of the nuclear cycle. In only a very 



246 



