1 82 FERTILIZATION OF THE OVUM 



essential phenomenon of fertilization is the union of a sperm-nucleus, 

 of paternal origin, with an egg-nucleus, of maternal origin, to form the 

 primary nucleus of the embryo. This nucleus, known as the cleavage- 

 or segmentation-nucleus, gives rise by division to all the nuclei of tJie 

 body, and hence every nucleus of the child may contain nuclear substance 

 derived from both parents. And thus Hertwig was led to the conclu- 

 sion ('84), independently reached at the same time by Strasburger, 

 Kb'lliker, and Weismann, that the nucleus is the most essential ele- 

 ment concerned in hereditary transmission. 



This conclusion received a strong support in the year 1883, through 

 the splendid discoveries of Van Beneden on the fertilization of the 

 thread-worm, Ascaris megalocephala, the egg of which has since ranked 

 with that of the echinoderm as a classical object for the study of cell- 

 problems. Van Beneden 's researches especially elucidated the struc- 

 ture and transformations of the germ-nuclei, and carried the analysis 

 of fertilization far beyond that of Hertwig. In Ascaris, as in all 

 other animals, the sperm-nucleus is extremely minute, so that at first 

 sight a marked inequality between the two sexes appears to exist 

 in this respect. Van Beneden showed not only that the inequality in 

 size totally disappears during fertilization, but that the two nuclei 

 undergo a parallel series of structural changes which demonstrate 

 their precise morphological equivalence down to the minutest detail ; 

 and here, again, later researches, foremost among them those of 

 Boveri, Strasburger, and Guignard, have shown that, essentially, the 

 same is true of the germ-cells of other animals and of plants. The 

 facts in Ascaris (variety bivalens) are essentially as follows (Fig. 

 90) : After the entrance of the spermatozoon, and during the for- 

 mation of the polar bodies, the sperm-nucleus rapidly enlarges and 

 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 (Fig. 90, 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 which has 

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

 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 maternal 

 chromatin. 



