154 



FERTILIZATION OF THE OVUM 



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. 1 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 union. 



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

 militate against the rule, the number of chromosomes arising from tJic 

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

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

 fore, the germ-nuclei give rise to an equatorial plate containing the 

 typical number 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. 



A PARTIAL LIST SHOWING THE NUMBER OF CHROMOSOMES CHAR- 

 ACTERISTIC OF THE GERM-NUCLEI AND SOMATIC NUCLEI IN 

 VARIOUS PLANTS AND ANIMALS. 2 



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

 germ-nuclei is exceptional. (Cf. p. 216.) 



2 The above table is compiled from papers both on fertilization and maturation. Num- 

 bers in brackets are inferred. 



