. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. 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, sorne 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 e
. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. 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, sorne 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, Zellcn-Studicn, 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 blastomeret; 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 types 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 { 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for rea
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