264 



INTERNAL FACTORS 



IV. i 



the number of which is 3 n x 2, where n is the reduced number 

 (in Echinus 9), since there are two male and one female 

 pronuclei and all the chromosomes have divided. Cell-division 

 occurs across each spindle and the whole breaks up simul- 

 taneously into four. To these four cells the 3^x2 chromo- 

 somes are distributed, and, as Boveri points out, the chance of 

 each cell obtaining a complete set of the n chromosomes of the 

 species (supposing these to be different from one another) is 



FIG. 161. Larvae from doubly fertilized eggs ; a, of Sphaerechhntfi 

 ;/><( nularis; b-d, of Echinus microtuberculatus ; , c, and d are from 

 tripartite, b from a quadripartite egg. (After Boveri, 1904.) 



small. Such eggs, as a matter of fact, practically always give 

 rise to abnormal larvae. Out of 1,170 examined not a single 

 one was normal. 



The chance, however, that one cell of the four will receive a com- 

 plete set of chromosomes is far greater, and when the | blastomeres 

 were isolated it was found that 25 % gastrulated, though none 

 gave rise to Plutei. Moreover, all four developed differently. 



