II CLEAVAGE 53 



value, whatever be the initial size of the nucleus, which may 

 be arbitrarily altered, while the cell-size is kept constant. 



The number of chromosomes can be altered in the following 

 ways: 



1. In artificial parthenogenesis the number is n (maternal) 

 (Thely kary otic) . 



2. In merogony the number is 7i (paternal) (Arrhenokaryotic). 



3. In monaster eggs the number is raised to 4-n (Diplo- 

 karyotic). These are fertilized eggs in which the division of 

 the centrosome has been delayed by shaking. The 2 n chromo- 

 somes nevertheless divide, but return into the condition of 

 a resting nucleus. From this, 4 n chromosomes emerge when 

 the centrosome does divide, and this number persists. 



4. If the egg is kept for twenty-four hours in sea-water 

 while the sperm is treated with dilute alkali, then upon fertili- 

 zation the sperm-nucleus lags behind its centrosome, which 

 divides to form a spindle in which only the female nucleus is 

 included. The latter breaks up into chromosomes which are 

 divided in the ordinary way, but the male nucleus passes un- 

 divided to one pole (Partial Thelykaryosis). Hence, after 

 cell-division, one blastomere has 2n chromosomes (is Amphi- 

 karyotic) while the other has only n, which are maternal (is 

 Thely kary otic). 



5. In dispermy it may happen that the spindles formed by 

 the division of the two sperm-centrosomes remain apart, with- 

 out uniting in the usual tetraster. The female nucleus lies in 

 the equator of one spindle, together with one male nucleus, the 

 other male nucleus lies in the other spindle. The two spindles 

 are parallel, or may be, and division takes place in the plane 

 including their equators, and also between them (if not at 

 once, then eventually). Hence on one side of the egg there 

 are Amphikaryotic nuclei with 2 n, on the other Arrhenokary- 

 otic with n chromosomes. This is Partial Arrhenokaryosis. 



6. The normal egg is Amphikaryotic, with 2 n chromosomes. 



The following examples will suffice to show how the rela- 

 tion between the number of chromosomes and the number 

 of the nuclei is determined, cell (cytoplasm) size remaining 

 constant. 



