Page:Popular Science Monthly Volume 61.djvu/48

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largely given to Weismann. The method of reduction imagined by him accounted for the origin of variation in the offspring, and thus furnished him with an explanation of the advantages, and therefore the existence of sexual reproduction. It was the attempt to verify or reject his theory which called forth so many monographs on this subject.

PSM V61 D048 Earliest stages of development of a new individual.png

Fig. 12. The Earliest Stages in the Development of the new individual. a. The separation of halves of the chromatic threads. b. Further separation and beginning of division of the egg. c. Complete separation and complete division of the egg. d. The two cells with their chromatic threads ready to divide again.

We shall not here go into details of the process of the reduction of the number of chromosomes. The most favorable objects for demonstrating many of the processes of maturation and fertilization are the reproductive cells of Ascaris. In Ascaris each of the nuclei is finally seen to contain two chromatic threads and the ultimate and intimate process of fertilization is seen from the photographs to be the union of these two groups of two chromosomes into one group of four chromosomes ready to divide. The dynamic agents in their union are two darkly stained bodies from which rays emanate, the centrosomes. In the present case one of these enters the egg with the sperm and this is the usual method of its origin in developing eggs. At the completion of the process of fertilization we have a cell containing a nucleus with the number of chromosomes normal to the species. One half of these chromosomes came from the father, one half from the mother. When this cell divides each one of the chromosomes divides longitudinally, and one half of each chromosome goes to one of the new cells and the other half to the other cell. The observations of Rückert, Hacker and Moenkhaus