134 



FERTILIZATION OF THE OVUM 



finally forms a typical nucleus exactly similar to the egg-nucleus. 

 The chromatin in each nucleus now resolves itself into two long, 

 worm-like chromosomes, which are exactly similar in form, size, and 

 staining reaction in the two nuclei. Next, the nuclear membrane 

 fades away, and the four chromosomes lie naked in the egg-substance. 

 Every trace of sexual difference has now disappeared, and it is 

 impossible to distinguish the paternal from the maternal chromo- 

 somes (Figs. 65, D, E). Meanwhile an amphiaster has been devel- 

 oped which, with the four chromosomes, forms the mitotic figure for 

 the first cleavage of the ovum, the chromatic portion of zuhich has 

 been syjithctically formed by the union of two equal germ-nuclei. The 



A 



B 



Fig. 66. — Germ-nuclei and chromosomes in tlie eggs of nematodes. [Carnoy.] 

 A. Egg of nematode parasitic in Scyllium ; the two germ-nuclei in apposition, each containing 

 four chromosomes; the two polar bodies above. B. Y^gg oi Filar aides ; each germ-nucleus with 

 eight chromosomes ; polar bodies above, deutoplasm-spheres below. 



later phases follow the usual course of mitosis. Each chromosome 

 splits lengthwise into equal halves, the daughter-chromosomes are 

 transported to the spindle-poles, and here they give rise, in the usual 

 manner, to the nuclei of the two-celled stage. Each of these nuclei, 

 therefore, receives exactly equal amounts of paternal and mater)ial 

 chromatin. 



These discoveries were confirmed and extended in the case of 

 Ascaris by Boveri and by Van Beneden himself in 1887 and 1888 

 and in several other nematodes by Carnoy in 1887. Carnoy found 

 the number of chromosomes derived from each sex to be in Coronilla 

 4, in Ophiostomum 6, and in Filaroides 8. A little later Boveri 



