104 DIFFERENTIATION III 



chromosomes, but the male are retarded and smaller, so that 

 while the daughter female chromosomes are passing to the 

 spindle-poles and becoming vesicular, the male are not so far 

 advanced and still lie about on the spindle. They may divide, 

 and their halves pass to the poles, but they often fail in this, 

 and then either pass undivided to one pole or the other, or 

 else get left behind to disintegrate in the cytoplasm. One 

 cell may therefore easily fail to obtain a full set of male 

 chromosomes, and the preponderance of female over male 

 chromatin in one cell accounts for the smaller nuclei and 

 preponderance of female over male characters on one side of 

 the larva. This is incompletely partial thelykaryosis. Com- 

 pletely partial thelykaryosis also occurs that is, all the male 

 chromosomes go into one cell and the larva has Sphaerechinus 

 characters on one side, hybrid on the other, with small nuclei 

 in the first and large in the second half. 



In the second case the female nucleus has advanced as far 

 as the monaster condition by the time that the spermatozoon 

 enters that is, the female chromosomes have divided and 

 there are radiations about the female nucleus. 



The female nucleus is now reconstituted from the chromo- 

 somes ; the male nucleus approaches but does not as a rule 

 unite with it. 



The female monaster now degenerates, the 2 n female 

 chromosomes reappear and are thrown on the equator of the 

 spindle formed in the meantime between the two sperm-centres. 

 The male nucleus may (a) break up into chromosomes which 

 lie either with or apart from the female chromosomes on the 

 spindle, or (ft) (and this is more usual) remain intact and lie on 

 the spindle-equator, near one spindle-pole, or remotely in the 

 cytoplasm. 



Should male chromosomes have been formed they are 

 dragged out as irregular strings and so divided (transversely) ; 

 if not, then the nucleus is either divided amitotically, or 

 else passes undivided into one blastomere. Except in the 

 last contingency, the paternal chromatin must be irregu- 

 larly distributed to the two blastomeres, and even when the 

 nucleus does pass entire to one cell and fuse with the female 



