. A manual of zoology. Zoology. GENERAL EMBRYOLOGY 143 nates {telolecithal eggs). Since the nuclear pole always becomes the ani- mal pole, there can be distinguished in the egg an animal part rich in pro- toplasm and a vegetative part rich in yolk. In many telolecithal eggs the two regions pass gradually into one another, but in others a distinct boundary separates an almost purely protoplasmic animal portion from a yolk-containing vegetative portion. This is well shown in the bird's egg (fig. 102). Here only the yolk is to be regarded as an egg in the embryo- logical sense, while the white, e
. A manual of zoology. Zoology. GENERAL EMBRYOLOGY 143 nates {telolecithal eggs). Since the nuclear pole always becomes the ani- mal pole, there can be distinguished in the egg an animal part rich in pro- toplasm and a vegetative part rich in yolk. In many telolecithal eggs the two regions pass gradually into one another, but in others a distinct boundary separates an almost purely protoplasmic animal portion from a yolk-containing vegetative portion. This is well shown in the bird's egg (fig. 102). Here only the yolk is to be regarded as an egg in the embryo- logical sense, while the white, egg-membrane, and calcareous shell are depositions upon the surface of the egg. The chief mass of the yolk is Fig. 102.—Diagrammatic longitudinal section through Balfour), (i) The egg: , blastoderm; , white yolk; Coverings of the egg: , yolk membrane (\'itelline membrane); .v and to., inner and outer layers of \\'hite; , chalazcc; and ^ inner and outer shell-membrane; between them at the right end is the air-chamber (); s, shell. a bird's egg (after , yellow yoUc. (2) deutoplasm, upon which rests a thin layer of protoplasm, the germinal disc, always uppermost in the bird's egg, whatever the position of the egg. The protoplasmic layer contains the egg-nucleus, and, after fertihzation, by progressive development is separated {blastoderm) more and more sharply from the underlying yolk. Types of Cleavage.—A brief explanation will now render the following figures of the various modes of cleavage intelligible. a. Holoblastic Eggs with Total Cleavage. I. Equal Cleavage.—The yolk, present only in small quantity, is distributed equally through the egg; upon cleaving, the egg divides into parts of approxi- mately the same size and equally rich in yolk {alecithal eggs, fig. loi).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these il
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