Page:Popular Science Monthly Volume 61.djvu/49

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43
THE PHYSICAL BASIS OF HEREDITY.

make it probable that this process is continued with every division, so that ultimately each cell of the adult contains chromosomes, one half of which are the lineal descendants of the chromosomes coming from the father, the other half lineal descendants of the chromosomes coming from the mother. Rückert has found that in a late stage of development in a crustacean the chromosomes were in two groups, presumably maternal and paternal. Moenkhaus has found that in crossing two species of fishes with PSM V61 D049 History of the reproductive process of the cymatogaster.pngFig. 13. History of the Reproductive Cells in Cymatogaster, from the Beginning of One Generation to the Beginning of the Next. 1 Fertilization of the egg. 2. Segmentation of the egg. 3. Segregation of the reproductive cells. 4. Period of inactivity. 5. Multiplication of the cells originally segregated. 6. Time of the differentiation of the sexes. 7. Continued multiplication of the cells in the female. 8. Period of growth of the individual cells. 9. Period of maturation. structurally and physiologically different chromosomes these retained their structural and physiological differences for a number of divisions. From the elaborate provisions to insure the union of the chromatic threads it is quite certain that they, finally, are the carriers of the hereditary power.

The character of any cell is controlled by the nucleus it contains, and, since we have seen that the nucleus contains two different groups of chromosomes, one group containing the peculiarities of the father and the other the peculiarities of the mother, the cause of the blending of the two sets of characters in the offspring becomes apparent and the greater resemblance in some characters to one parent and in other characters to the other parent may readily be inferred.

Without attempting to review the recent speculations and observations on the origin of the hereditary cells, I want to give an outline of some observations I made about ten years ago, and which have recently been confirmed by Beard. Very early in the development of one of the California viviparous fishes certain cells apparently lose their interest in the development. They undergo very little change, while the rest of the cells are busily engaged in multiplying and forming themselves into the various organs of the young fish. These cells become shifted somewhat and probably engage in active migrations. Late in the development they again begin to divide and ultimately give