Archive image from page 178 of Cytology, with special reference to. Cytology, with special reference to the metazoan nucleus cytologywithspec00agar_0 Year: 1920 VI DISPERMIC EGGS 163 have just discussed, for experimental embryology has established the fact of the local differentiation of the cytoplasm of the egg (the so-called ' organ-forming ' substances). This matter is discussed on pp. 188, etc. We have already found morphological evidence of the differentiation of the chromatin in the genetic continuity of the chromosomes, the constant size differences often visible between them, their c
Archive image from page 178 of Cytology, with special reference to. Cytology, with special reference to the metazoan nucleus cytologywithspec00agar_0 Year: 1920 VI DISPERMIC EGGS 163 have just discussed, for experimental embryology has established the fact of the local differentiation of the cytoplasm of the egg (the so-called ' organ-forming ' substances). This matter is discussed on pp. 188, etc. We have already found morphological evidence of the differentiation of the chromatin in the genetic continuity of the chromosomes, the constant size differences often visible between them, their composition out of chromomeres of different but constant sizes, etc., and it is a necessary corollary of the hypothesis of the dependence of Mendelian phenomena upon the chromosomes (see below under (D)). Direct experi- mental evidence of the functional differentiation of the chromosomes has been obtained by Boveri (1907) from his remarkable experiments on the development of polyspermic Echinoderm eggs. Among normally fertilized Echinoid eggs it occasionally happens Fig. 74. Tnaster and tetraster mitotic figures from a sea-urchin, Strongylocentrotus lividus. (After Baltzer, Verh. Phys. Med. Gesells, Wurzburg, igo8.) that two spermatozoa enter the egg instead of one. By increasing the concentration of sperm, the percentage of such dispermic (or poly- spermic) eggs can be enormously increased. Thus in two parallel experi- ments, eggs placed in water with only a few spermatozoa resulted in a hundred normal monospermic and no di- or polyspermic fertilizations. On the other hand, eggs placed in very concentrated sperm gave only eleven monospermic and eighty-nine di- or polyspermic fertilizations. When an Echinoid egg is fertilized by two spermatozoa both sperm nuclei (typically) fuse with the egg nucleus, and the centrosome intro- duced by each spermatozoon divides as if it were the only one—hence we get a zygote nucleus with 311 (=54) chromosomes and four centro- somes. A four-
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