THE INDIVIDUALITY OF THE GERM-NUCLEI DURING THE CLEAV- 

 AGE OF THE EGG OF CRYPTOBRANCHUS ALLEGHENIENSIS. 



A Preliminary Report.* 



BERTRAM 0. 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. Later it was found that in many cases, though appar- 

 ently not in all, the two germ-nuclei merely become opposed without actual 

 fusion. In tracing their further history, attention early became focused upon 

 the chromosomes, and 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 

 cells. "Later observations have given the strongest reasons for believing that, 

 as far as the chromosomes are concerned, a true fusion of the nuclei never 

 takes place during fertilization, and that the paternal and maternal chromatin 

 may remain separate and distinct in the later stages of development — possibly 

 throughout life." (Wilson, 1900). At the present time we may state with 

 some confidence that a fusion of maternal and paternal chromatin never takes 

 place in the somatic cells, and that in the lineage of germ cells it does not 

 occur until a pairing of maternal and patei'nal chromosomes, called synapsis, 

 takes place in preparation for the maturation divisions. In a sense, the process 

 of fertilization is not complete until synapsis, when for the first time maternal 

 and paternal chromosomes are brought together in intimate and orderly union, 

 in some cases amounting to actual fusion. 



This conclusion, which is of the most fundamental importance since it 

 vitally concerns the mechanism of inheritance, is in part based upon indirect 

 evidence. In the attempt to establish the principle by direct demonstration, 

 the chief difficulty has been encoimtered in tracing the maternal and paternal 

 chromosome-complexes through the resting stage of the nucleus. In only a 

 very few cases has this been done with even partial success ; the observations 

 of Haecker (1892 and 1895), Ruckert (1895) and Conklin (1901) are of most 

 importance in this connection. 



In the developing egg of the amphibian Cryptotranchus allegheniensis I 

 have found material very favorable for the study of this problem; for the 

 early cleavage nuclei are almost without exception distinctly double throughout 

 the entire resting stage, each consisting of two separate nuclear vesicles, and 

 the origin of these two components from the maternal and paternal germ- 

 nuclei respectively has been traced through several of the early nuclear divi- 



•Illustrated by lantern slides and microscopical demonstrations. 

 21st Mich. Acad. Sci. Kept, 1919. 



