. The cell in development and inheritance. Cells. 296 SOME PKOBLEjMS OF CELL-ORGANIZATION C, D). Again, the two chromosomes left in the egg after removal of the second polar bod}- may accidentally become separated. In this case each chromosome gives rise to a reticular nucleus of half the usual size, and from each of these a single chromosome is afterward formed (Fig. 143, A, B). Finally, it sometimes happens that the two germ-nuclei completely fuse, while in the reticular state, as is normally the case in sea-urchins and some other animals (p. 188). - From the cleavage-nucleus thus formed ari
. The cell in development and inheritance. Cells. 296 SOME PKOBLEjMS OF CELL-ORGANIZATION C, D). Again, the two chromosomes left in the egg after removal of the second polar bod}- may accidentally become separated. In this case each chromosome gives rise to a reticular nucleus of half the usual size, and from each of these a single chromosome is afterward formed (Fig. 143, A, B). Finally, it sometimes happens that the two germ-nuclei completely fuse, while in the reticular state, as is normally the case in sea-urchins and some other animals (p. 188). - From the cleavage-nucleus thus formed arise four chromosomes. The same general result is given by the observations of Zur Strassen ('98) on the history of giant embryos in Ascai'is. These embryos arise by the fusion, cither before or after the fertilization, of previ- ously separate eggs, and have been shown to be capable of develop- ment up to a late stage. Not only in the first but also in some, at least, of the later mitoses, such eggs show an increased number of chromosomes proportional to the number of nuclei that have united. Thus in monospermic double eggs (variety bi- va/ens) the. number is six instead of four ; in dispermic double eggs the number is increased to eight (Fig. 144). These remarkable observations show that zvJiatcvcr be the number of ehronio- somes enterijig into the formation of a reticular nucleus, the same number after- %vard issues from it — a result which de- monstrates that the number of chromo- somes is not due merely to the chemical composition of the chromatin-substance, but to a morphological organization of the nucleus. A beautiful confirmation of this conclusion was afterward made by Boveri ('93, '95, i) and Morgan ('95, 4), in the case of echinoderms, by rear- ing larvae from enucleated egg-fragments, fertilized by a single sper- matozoon (p. 194). All the nuclei of such larvae contain but half the typical number of chromosomes, — in EcJiinus nine instead of eighteen, — si
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