VI 



DISPERMIC EGGS 



165 



of monospermic eggs, which, as we have just seen, develop practically 

 normally. The same applies in principle to the triaster egg. 



Boveri found, however, that if the blastomeres of tetraster eggs are 

 separated, some or all of them develop abnormahties which lead to 

 their early death, while very rarely, or never, do they all grow into normal 

 plutei. Moreover, and this is very important, the abnormahties appearing 

 in the four embryos which are derived from the four separated blastomeres 

 of a single egg are often of quite different types. Thus, to take one 



Fig. 75. 



Diagram of distribution of chromosomes in the first cleavage division of dispermic sea-urchin eggs. (Boveri, 

 Zcllen-Studien, 1907.) A, B, C, showing one of the many possible arrangements of the 54 chromo- 

 somes on the tetraster spindle figure, and the consequent distribution of the 108 daughter chromosomes between 

 the four blastomeres ; D, a possible arrangement of the three sets of chromosomes belonging to the three gametic 

 nuclei (only 4 — designated a, b, c, d — out of the 18 chromosomes of each gamete are shown) ; E, the four 

 blastomeres resulting from D. Only the cell in the bottom left-hand comer has a representative of each of the 

 four tj'pes of chromosomes, and therefore it is the only cell that can develop normally. 



example from the twenty-one given by Boveri, the four blastomeres of 

 one dispermic egg gave : 



One good gastrula. 



One very thick-walled stereoblastula {i.e. blastula .with blasto- 

 coele filled with cell masses). 



One compact clump of cells. 



One heap of isolated cells. 

 Similar differences were found amongst the larvae developed from isolated 

 blastomeres of triaster feggs, only here a far greater proportion of them 

 developed normally. This, it is to be noted, is in agreement with the 



