A text-book of embryology for students of medicine / by John Clement Heisler.

  • Heisler, John Clement.
Date:
1907
Catalogue details

Licence: In copyright

Credit: A text-book of embryology for students of medicine / by John Clement Heisler. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    nuclear spindle, a second one is formed, which in the same manner extrudes the second polar body. What remains of the nucleus now moves toward the center of the cell and is known as the female pronucleus. The position of the female pronucleus is nearly or absolutely central. The egg is now ready for fertilization. Fig. 13.—A, mature ovum of echinus; n, female pronucleus; B, immature ovarian ovum of echinus (Hertwig). For some time after their extrusion, and pending their final disappearance and disintegration, the polar bodies are to be seen lying in the perivitelline space. The formation of polar globules is probably almost universal throughout the animal world. It is of interest to note that in some par- thenogenetic eggs—that is, eggs capable of developing into a new individual without contact with the male element, as, for example, the summer eggs of plant lice and of some other arthropods—only one polar globule is said to be formed, and it has recently been shown (Sobotta) that in the maturation of the ovum of the mouse only one polar body was formed in the majority of cases. The maturation of the ovum is essentially a reduction of the chromosomes precisely analogous to the reduction-division seen in the descent of the spermatozoon from the primary spermatocyte. The last generation of oogonia having in- creased in size after their stage of rest, and having thus become the ovarian eggs or primary oocytes, now undergo mitosis, but in a manner differing from that of their pred- ecessors. The chromatin thread of the nucleus, instead of