4 TEXT-BOOK OF EMBRYOLOGY. 



delicate vitelline membrane, not easily seen, surrounds each ovum. This 

 is a true cell membrane, a product of the egg cytoplasm. Outside of this is a 

 tough membrane called the chorion and then the gelatinous capsule, both 

 being secondary egg membranes produced by the cells of the oviduct and 

 not by the cytoplasm of the ovum. 



If the egg is bisected through the centers of the dark and light areas the 

 two halves are exactly alike. The cut surface of either half shows three 

 substances: pigment, cytoplasm and yolk. The pigment forms a superficial 

 layer which coincides with the dark superficial area. It is a product of 

 cytoplasmic activity without any known importance in future development. 

 The portion of the egg not covered by pigment contains a large amount of 

 yolk, in fact more yolk than cytoplasm, in the form of globules of different 

 sizes. The remainder of the egg contains some yolk but the cytoplasm is 

 excessive. Therefore we may speak of the cytoplasmic or animal pole and 

 the yolk or vegetal pole of the egg, the former approximately indicated on 

 the surface by the dark area and the latter by the light area. The yolk has 

 a slightly higher specific gravity than the cytoplasm, which accounts for the 

 fact that if the egg is left free in its natural medium the dark pole turns up- 

 ward. An egg like this in which more yolk is accumulated at one side than 

 at the other is known as a telolecithal ovum as distinguished from one of the 

 homolecithal type in which the yolk granules are distributed uniformly or 

 nearly so, as in the mammalian ovum. 



The nucleus of the frog's ovum is proportionately smaller than in the 

 case of an egg with a small quantity of yolk. It is conspicuously eccentric, 

 situated nearer the animal than the vegetal pole. Being thus situated it 

 obviously tends to occupy the center of the cytoplasmic mass. The nuclear 

 membrane encloses the usual nuclear components; the chromatin is rather 

 scanty and numerous small nucleoli (plasmosomes) are present. 



The freshly laid hen's egg may be chosen as an example of a large ovum 

 with a relatively great quantity of yolk (Fig. 3) . The shape is characteristic. 

 The outer covering is the shell, a calcareous substance. If the shell is broken 

 the tough shell-membrane appears; this is a double layer with a considerable 

 air space between the layers at the larger end of the egg. Enclosed by this 

 membrane is the thick layer of albuminous substance with a denser twisted 

 portion, the chalaza, at each end of the egg. All these structures are second- 

 ary egg membranes secreted around the ovum proper by the epithelium of 

 the oviduct during its passage through that organ. 



The ovum proper consists of the large spherical mass of yolk, 25 mm. 

 or more in diameter, and a small disk of cytoplasm, 3 or 4 mm. in diameter, 

 which rests upon the yolk. If the unbroken egg is allowed to lie in one 

 position for a minute or two the disk will be found uppermost when the shell 



