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

  • Heisler, John Clement.
Date:
1907
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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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    dant deutoplasm is accumulated at one side of the ovum, called the vegetative pole, while the protoplasm appears as a flat germ-disk at the animal pole on the opposite side. Here belong the eggs of birds, reptiles, and bony fishes (see Fig. 6). 3. Centrolecithal ova are those in which the deutoplasm is central, the protoplasm completely surrounding it, as in the eggs of arthropods (Fig. 7). Ova are classified also according to their method of seg- mentation. This will be described later. The germinal vesicle or nucleus is the most important part of the cell, since, as will be seen hereafter, it is essentially by the conjugation, or more accurately by the fusion, of the nuclei of the male and female parent-cells that generation is effected. As a rule, there is but one nucleus, though there may be two. Its position is usually—if not universally— eccentric, this being more marked where there is a distinct differentiation into animal and vegetative poles, in which case it is found always near the animal pole. It is nearly spheri- cal in shape, and like the nucleus of any other typical cell, it is composed of the nuclear network consisting of linin and chromatin, and nuclear juice or achromatin, the former con- taining the latter within its meshes. Surrounding the nucleus is the well-marked nuclear membrane, wThile within it is the nucleolus or germinal spot. The latter may be single or multiple, according to the species, though the number is fairly constant for each species. Nagel ascribes ameboid move- ment to the germinal spot. Polarity.—The polarity of the egg has been incidentally referred to. Apparently it owes its existence to the eccentric position of the nucleus, the animal pole being that point on the surface to which the nucleus is nearest. Polarity bears a significant relation to the specific gravity of the ovum, since the nucleus reaches the surface of the latter at the animal pole and there extrudes the polar globules; and it is also related to the segmentation of the fertilized egg. The Hen's Egg.—As the hen's egg is so largely utilized for the study of development, it will be profitable to consider briefly its structure. The ovum or egg-cell is represented by
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