Ill DIFFERENTIATION 91 



6. They remain apart. 



The first and third types are those which immediately 

 interest us. In these tetracentric and tricentric, simul- 

 taneously quadripartite and tripartite ova, the 3n chromo- 

 somes divide and the 6n chromosomes are then thrown at 

 random on the equators of the spindles connecting the various 

 centres. In the anaphase of the mitosis the daughter chromo- 

 somes pass to the four or three centres, and the four or three 

 cells consequently receive a random number of the elements. 

 Assuming for the moment that the chromosomes are qualita- 

 tively unlike, each cell also receives a perfectly random assort- 

 ment of them, and the chance of every cell receiving at least 

 one of each kind of chromosome is very small indeed, but 

 greater of course for the tripartite than for the quadripartite 

 ova. The chance, however, that one cell will receive a full 

 complement is much larger. 



A study of the development shows that the quadripartite 

 practically never develop normally, the tripartite sometimes 

 but not often, while, if the four cells of the former or the 

 three cells of the latter be isolated and allowed to develop 

 independently, a very fair percentage of normal larvae is 

 obtained. All four, or three, develop differently. In the first 

 case one or even two may give normal larvae, but never all 

 four, while of the three cells of the triasters one or two, rarely 

 all three, reach the pluteus stage. 



The same differences between the blastomeres are seen when 

 they develop in connexion with one another. 



The tripartite ova gave a mean of 8% normal larvae, out of 

 828 reared in all. 



The normal larva consists of three regions, marked by the 

 different size of the nuclei, dependent on the different number 

 of chromosomes in the original three cells. The egg-axis, 

 segmentation-axis, and gastrula-axis being all coincident, the 

 boundaries between the three regions meet in the blastopore 

 at one end, the animal pole (anterior end) at the other. One 

 boundary is generally in the median longitudinal plane. 



Special cases are (1) where the nuclei are not of different 

 sizes and the regions therefore indistinguishable; (2) where 



