238 KIYOSHI ILiSÜI. 



These facts lead us to the conclusion that in the mouse the conjugation 

 of chromosomes may occur by telosjiiapsis. 



In the rabbit the ring shaped chromosomes ap^iear to be formed by the 

 separation of the vmivalent chromosomes along the plane of a longitudinal 

 spHtting. It has, however, been found that this, in fact, is not the case, the 

 sphtting seen in the chromosomes probably represents the line of conjugation 

 of the univalent chromosomes, since the splitting apjiears only in the special 

 chromosomes. From these facts it is more probable that the conjugation of the 

 chromosomes in the rabbit takes place by parasynapsLs. 



Although the method of synapsis in the mouse is diiferent from that in the 

 rabbit, in both these auimaJs, the conjugated chromosomes Ijecome disjointed. 

 Thus by wluiteyer method of the synapsis the conjugation of chromosomes may 

 occur, it will aime to the same result as regards the reduction of the 

 chromosomes. 



m. The Sex-Chromosomes and the Nucleolus. 



The sex-chromosomes : — As far as I am aware, sex-chromosomes in the 

 nxlouts were fii-st recorded by Stevens ('12) in the guinea-pig. This was 

 followed by Jobdan ('14) in the mouse, by Bachhuber ('16) in the rabbit 

 and by Allen ('18) in the rat. 



Jordan ('14) in the white mouse found the double nature of the hetero- 

 chi'omosome which suggests a pair of idiochromosomes, but he boheved this 

 to be a double accessory chromosome or Wilson's X-chi'omosome. In the 

 rabbit he found that heteroehromosomofj are wanting in the spermatocyte of 

 the growth stage. In the male germ cell of the mouse YocuM ('17) found 

 the accessory chromosome which does not divide in the primary division, but 

 does sa in the secondary. As stated above in almost every stage of the 

 spermatogenesis in the mouse XY- or idochromosomes usually appear wldch 

 correspond almost exactly with those of the insects (Wilson) and the guinea-pig 

 (Stevens). 



In the rabbit the accessory or the X-chromosome can be seen in the 

 grow til pericxl of the spermatocyte and in the reduction division, but the 

 occurrence of the dimorphism of the spermatozoa as regartls to the existence 

 of tills chi-omosome can not practically be determined. 



