72 



THE REALISATION OF THE NUCLEAR FACTORS 



to Boveri, is due to the fact that each of the penetrating sperms 

 has introduced a centrosome into the egg. One, or more often 

 both, of these centrosomes will divide into two, giving rise 

 to a total of three or four centrosomes which then will constitute 

 the poles of a three- or four-polar mitosis. The haploid chromo- 

 some number of these sea urchins is 18. The triploid zygote nucleus 

 therefore contains 54 chromosomes. Each of them doubles it- 

 self, and the members of each pair move apart to the neigh- 

 bouring poles of the spindle (Fig. 27). As the distribution of 

 the chromosomes over the three or four spindles is entirely 

 random, it is a matter of chance how many chromosomes will 

 arrive at each of the poles, and combine to 

 form a daughter-nucleus. The average number 

 will be 108 : 4=27 in the case of a four-polar, 

 and 108 : 3=36 in that of a three-polar mitosis, 

 but the numbers may vary widely in individual 

 cells. 



By two different methods Boveri made a care- 

 ful study of the further development of eggs 

 after double fertilisation. Sometimes he allowed 

 complete eggs to develop, but at other times 

 he separated the three or four simultaneously 

 formed daughter-cells by bringing them into 

 calcium-free sea water (this simple method 

 for the isolation of the blastomeres was dis- 

 covered by Herbst). Boveri found that, on 

 the whole, the isolated cells developed poorly, 

 but that there were great individual differ- 

 ences. Some cells died very soon, others devel- 

 oped better and produced fairly normal small 

 larvae. Experiments with whole eggs had the 

 same result. As a rule, cleavage was normal, 

 but then development came to a stop in certain 

 sectors of the egg, and the cells of those 



Fig. 27. Diagram of a division of the tetraster type in a sea urchin 

 egg after double fertilisation; the numbers indicate the distribution 

 of the chromosomes over the various spindles and daughter nuclei. 



After Stomps. 



