UNION OF THE GERM-CELLS 



205 



cerned, a true fusion of the nuclei never takes place during fertili- 

 zation, and that the paternal and maternal chromatin may remain 

 separate and distinct in the later stages of development — possibly 

 throughout life (p. 299). In this regard two general classes may be 

 distinguished. In one, exemplified by some echinoderms, by Aviplti- 

 oxus, Phallnsia, and some other animals, the two nuclei meet each 

 other when in the reticular form, and apparently fuse in such a manner 

 that the chromatin of the resulting nucleus shows no visible distinc- 

 tion between the paternal and maternal moieties. In the other class, 

 which includes most accurately known cases, and is typically repre- 

 sented by Ascaris (Fig. 90) and other nematodes, by Cyclops (Fig. 98), 

 and by Pterotrachea (Fig. 93), the two nuclei do not fuse, but only 

 place themselves side by side, and in this position give rise each to 

 its own group of chromosomes. On general grounds we may confi- 

 dently maintain that the distinction between the two classes is only 

 apparent, and probably is due to corresponding differences in the rate 

 of development of the nuclei, or in the time that elapses before their 

 union. ^ If this time be very short, as in echinoderms, the nuclei 

 unite before the chromosomes are formed. If it be more prolonged, 

 as in Ascaris, the chromosome-formation takes place before unicm. 



With a few exceptions, which are of such a character as not to 

 militate against the rule, tJie number of cJiromosomcs arisitij^ f'om tJic 

 germ-nuclei is always the same in botJi, and is one-half the number 

 characteristic of the tissue-cells of the species. By their union, there- 

 fore, the germ-nitclei give rise to an equatorial^ plate eonfanung the 

 typical niLuiber of chromosomes. This remarkable discovery was first 

 made by Van Beneden in the case of Ascaris, where the number of 

 chromosomes derived from each sex is either one or two. It has 

 since been extended to a very large number of animals and plants, a 

 partial list of which follows. 



1 Indeed, Boveri has found that in Ascaris both modes occur, though the fusion of the 



germ-nuclei is exceptional. (^Cf. p. 296.) 



