1 82 FERTILIZATION OF THE OVUM 



essoitial pJuiiovioiou of fcrtilizatioi is tJic union of a spciin-uucItHSy 

 of paternal origin, ivith an cgg-nuclcns, of maternal origin, to form tJie 

 primary niielens of the emluyo. TJiis nucleus, knozi'n as tJie cleavage- 

 or segmentation-nucleus, gives rise by divisioii to all t/w nuclei of the 

 body, and hence ever]' nucleus of the child )nay contain nuclear substance 

 y' derived from both parents. And thus Hcrtwi<^ was led to the conclu- 

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

 Kolliker, and Weismann, that the nucleus is the most essential cle- 

 ment concerned in hereditary transmission. 



This conclusion received a strong suj^port in the year 1883, through 

 the s]:)lendid discoveries of Van Heneden on the fertilization of the 

 thread-worm, Ascaris megalocephala, the <t^g 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 totallv disappears during fertilization, but that the two nuclei 

 undergo a parallel series of structural changes which demonstrate 

 their precise morj:)hological 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 chronmtic portion of zuhicJi lias 

 been synthetically formed by the unioii 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. EacJi of these nuclei, 

 tJici'cfore, receives exactly equal amounts of paternal and maternal 

 chromatin. 



