tozoön is about to pierce, the yolk is drawn out into a conical elevation, termed the cone of attraction. As soon as the spermatozoon has entered the yolk, the peripheral portion of the latter is transformed into a membrane, the vitelline membrane which prevents the passage of additional spermatozoa. Occasionally a second spermatozoön may enter the yolk, thus giving rise to a condition of polyspermy: when this occurs the ovum usually develops in an abnormal manner and gives rise to a monstrosity. Having pierced the yolk, the spermatozoön loses its tail, while its head and connecting piece assume the form of a nucleus containing a cluster of chromosomes. This constitutes the male pronucleus, and associated with it there are a centriole and centrosome. The male pronucleus passes more deeply into the yolk, and coincidently with this the granules of the cytoplasm surrounding it become radially arranged. The male and female pronuclei migrate toward each other, and. meeting near the center of the yolk, fuse to form a new nucleus, the segmentation nucleus, which therefore contains both male and female nuclear substance; the former transmits the individualities of the male ancestors, the latter those of the female ancestors, to the future embryo. By the union of the male and female pronuclei the number of chromosomes is restored to that which is present in the nuclei of the somatic cells.
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Fig. 9.—First stages of segmentation of a mammalian ovum. Semidiagrammatic. (From a drawing by Allen Thomson.) z.p. Zona striata, p.gl. Polar bodies, a. Twocell stage, b. Four-cell stage, c. Eight-cell stage. d, e. Morula stage.
SEGMENTATION OF THE FERTILIZED OVUM.
The early segmentation of the human ovum has not yet been observed, but judging from what is known to occur in other mammals it may be regarded as certain that the process starts immediately after the ovum has been fertilized, i. e., while the ovum is in the uterine tube. The segmentation nucleus exhibits the usual mitotic changes, and these are succeeded by a division of the ovum into two cells of nearly equal size.[1] The process is repeated again and again, so that
- ↑ In the mammalian ova the nutritive yolk or deutoplasm is small in amount and uniformly distributed throughout the cytoplasm; such ova undergo complete division during the process of segmentation, and are therefore termed holoblastic. In the ova of birds, reptiles, and fishes where the nutritive volk forms by far the larger portion of the egg the clevage is limited to the formative yolk, and is therefore only partial; such ova are termed meroblastic. Again, It has been observed, in some of the lower animals, that the pronuclei do not fuse but merely lie in apposition. At the commencement of the segmentation process the chromosomes of the two pronuclei group themselves around the equator of the nuclear spindle and then divide; an equal number of male and female chromosomes travel to the opposite poles of the spindle, and thus the male and female pronuclei contribute equal shares of chromatin to the nuclei of the two cells which result from the subdivision of the fertilized ovum
