132 FERTILIZATIOX OF THE OVUM 



I. The Gcnu-iiiiclci in Frrti/i. 'nation 



The modern era in the study of fertilization may be said to begin 

 with Oscar Hertwig's discovery, in 1875, of the fate of the sperma- 

 tozoon within the egg. Earlier observers had, it is true, paved the 

 way by showing that, at the time of fertilization, the Qgg contains 

 Hvo nuclei that fuse together or become closely associated before 

 development begins. (Warneck, Biitschli, Auerbach, Van Beneden, 

 Strasburger.) Hertwig discovered, in the O-gg of the sea-urchin 

 ( Toxopnciistcs lividiis), that one of these nuclei belongs to the egg, 

 ivhile the other is derived from the sperniatozodn. This result was 

 speedily confirmed in a number of other animals, and has since been 

 extended to every species that has been carefully investigated. The 

 researches of Strasburger, De Bary, Schmitz, Guignard, and others 

 have shown that the same is true of plants. In every hnoivn ease an 

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

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

 primary nnclens of the embryo. This nucleus, kjiozvn as the cleavage- 

 or segmentation-7iucleus, gives rise by division to all the nuclei of the 

 body, and hence every nnclens of tJie 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, 

 Kolliker, 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 mcgalocepJiala, 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 struct- 

 ure 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 

 sjze 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. 

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

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



