Archive image from page 173 of Cytology, with special reference to. Cytology, with special reference to the metazoan nucleus cytologywithspec00agar_0 Year: 1920 majority die. Those that reach the pluteus stage are found to exhibit exclusively the characters of the female parent (Strongylocentrotih). Cytological examination of these hybrids (Strongylocentrotus $ x Sphaerechinus <$) pro- vides a satisfactory ex- planation both of the pathological course of development and of the purely maternal char- acteristics of the plutei. In the anaphase of the first cleavage division a number of chrom
Archive image from page 173 of Cytology, with special reference to. Cytology, with special reference to the metazoan nucleus cytologywithspec00agar_0 Year: 1920 majority die. Those that reach the pluteus stage are found to exhibit exclusively the characters of the female parent (Strongylocentrotih). Cytological examination of these hybrids (Strongylocentrotus $ x Sphaerechinus <$) pro- vides a satisfactory ex- planation both of the pathological course of development and of the purely maternal char- acteristics of the plutei. In the anaphase of the first cleavage division a number of chromosomes fail to travel up to the poles with the other chromosomes, but remain lying in between the two daughter groups (Fig. 72). They fail to enter into the daughter nucleus, re- maining as extra-nuclear masses of chromatin in the blastomeres. A further number of chro- mosomes get left out in the second cleavage mitosis. The nuclei from now onwards have only about twenty or twenty- one chromosomes (vary- ing in different larvae from nineteen to twenty- four) instead of the expected thirty-six (n in both parent species being eighteen). Thus about fifteen chromosomes have been eliminated. These rejected chromosomes are, in all probability, those brought in by the male gamete. For instance, two pairs of the Sphaerechinus and one pair of the Strongylocentrotus chromosomes are distinguishable by their shapes and sizes. After the elimination we never find these two Sphaerechinus chromosomes, but the â \ ⢠/ 'W- v. Li J r ⢠& 1\\ B V Fig. 72. The chromosomes of the hybrid Strongylocentrotus 9 x Sphaer- echinus <$. (Baltzer, , 1910.) A, B, anaphase and telophase of the first cleavage division. A number of chromosomes fail to enter either daughter nucleus. C, 4-cell stage. The eliminated chromosomes form irregular masses of chromatin between the daughter nuclei.
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