William J. Moenkhaus 41 



tering upon this, however, for reasons to be stated afterwards, it will be 

 important to give a brief general account of the character of the work 

 thus far done on this subject. 



4. General Review of Literature.— Since the first discoveries by van 

 Beneden, 83, Boveri, 90, Guignard, 91, and others, of the numerical 

 equality of the maternal and paternal chromosomes in fertilization, mucii 

 interest has developed in the question whether these might not retain 

 their individuality throughout all the cells of the developing embryo. 

 Van Beneden, who worked with Ascaris, in which the pronuclei may not 

 fuse before the formation of the first cleavage spindle, was able to follow 

 the maternal and paternal chromosomes into the resting nucleus of the 

 first two daughter cells. Here they were lost. Although unable to fol- 

 low them beyond the first cleavage, he expresses his conviction that the 

 two chromatin masses probably remain distinct throughout subsequent 

 divisions. Boveri, 91, working with the same animal, made similar ob- 

 servations and was led to formulate his well-known hypothesis " that in 

 all cells derived in the regular course of division from the fertilized egg, 

 one-half of the chromosomes are of strictly paternal and the other half 

 of maternal origin." He further endeavored to follow out the fate of 

 the individual chromosome during the resting period of the nucleus. 

 Boveri differed from van Beneden in this important respect, that he found 

 that not only the maternal and paternal chromatin remained distinct but 

 also that the individual chromosomes retained their individuality. With 

 this interesting and important question thus clearly pointed out so long 

 ago, one should consider it remarkable that so few researches have siru:e 

 been directed toward its solution, were it not for the evident difficulties 

 attending any effort to distinguish the exactly similar parental chromo- 

 somes beyond the first cleavage. Extension of our knowledge to a large 

 number of forms showed that three conditions obtained in regard to the 

 fusion of the pronuclei during fertilization : (1) Animals in which the 

 two pronuclei are so completely fused as no longer to be distinguishable. 

 (2) Animals in which the pronuclei do not fuse but remain more or less 

 separated by a membrane. (3) Animals in which both conditions may 

 occur. 



In 1893 Hacker pointed out that in Cyclops tenuicornis also, the two 

 pronuclei do not fuse in fertilization and, furthermore, that in the two- 

 cell stage the nuclei are composed of two closely united but distinct 

 halves, one of which he identifies with the male, the other with the female 

 pronucleus. 



Eiickert, 95, extended these observations to Cyclops strenuous and pub- 



