1 8 PROBLEMS OF FERTILIZATION 



of cell division was accurately ascertained and applied 

 to the genealogy of the germ cells, until the respective 

 parts of ovum and spermatozoon in the origin of the new 

 generation were understood, nor until the hoary doctrine 

 of spontaneous generation was banished bodily from the 

 field of biology. These were all accomplishments of 

 that great decade in biological research (1870-80), 

 for which the studies of the preceding thirty years had 

 furnished ample preparation. The entire superstructure 

 of modern genetic research rests upon the foundations 

 then laid. 



Professor Mark's paper on Limax (1881) is a point 

 of departure between the fertilization studies of the 

 seventies and those that were to follow. Professor 

 Mark observed that the pronuclei come together but 

 do not fuse to form a first cleavage nucleus, as had been 

 described for other animals. " The first cleavage nucleus 

 does not have a morphological existence.' The pro- 

 nuclei persist after the appearance of the cleavage 

 centers; their membranes then gradually disappear. 



In 1883 Van Beneden published his now classic 

 paper on Ascaris. The pronuclei do not fuse; both are 

 included in a single amphiaster; each produces two 

 chromosomes; these divide and their halves form the 

 daughter-nuclei. In the nuclei of the first two cells there 

 are thus equal numbers of male and female elements; 

 and there are reasons to believe that even in these two 

 nuclei they do not fuse ; it is probable that they remain 

 distinct in all derivative cells, including the immature 

 eggs and spermatogonia. In the egg the chromatin 

 is composed of two distinct parts, and "it is legitimate 

 to suppose that each is the equivalent of a male and a 



