OOGENESIS AND EARLY EMBRYOLOGY ASCARIS 559 



be resolved into its component chromosomes (figs. 47, 48, and 52), 

 which Ue imbedded in a matrix of non-chromatic material. This 

 matrix gradually fades as the nuclear membrane is formed, 

 leaving the chromosomes (fig. 53) lying free within the karyo- 

 plasm. Each chromosome is dyad-like in form, not differing in 

 appearance from the ones which united to form the compact 

 nuclear mass of chromatin in the late spermatids. 



2. The development of the male and female pronuclei 



At this stage (fig. 53) the two pronuclei are of exactly the 

 same appearance, each having eighteen dyad-like chromosomes. 

 Though alike, the two pronuclei can be distinguished by their 

 positions in relation to the polar bodies and the centrosomes. 

 The female pronucleus lies near the place where the second polar 

 cell was formed, while the male pronucleus generally is close to 

 the centrosomes, which, as we have seen, are of male origin. 

 These aids in distinguishing between the male and female pro- 

 nuclei are necessary only when the sperm nucleus contains 

 eighteen chromosomes, namely the heterochromosome complex 

 of six idiosomes in addition to the twelve autosomes. When 

 fertilization is accomplished by a sperm lacking the six idio- 

 somes, the male pronucleus, containing only the twelve auto- 

 somes (fig. 54), is easily distinguishable from the female pro- 

 nucleus with its eighteen chromosomes. 



The development of both types of male pronuclei is so similar 

 that the details will be followed in only the one containing the 

 heterochromosome group. 



The male pronucleus is slightly more precocious in its de- 

 velopment than the female (fig. 55). The chromosomes of the 

 former break up into small granules, which are grouped into a 

 single mass near the nuclear membrane; from this mass radiate 

 many fine linin fibers. There appear from two to twelve non- 

 chromatic nucleoli, or plasmosomes, which apparently are en- 

 tirely unconnected with the linin meshwork. The granular 

 mass of chromatic material (fig. 55) soon becomes less granular 

 (fig. 56), and looks like a tangle of short threads. 



