686 EGG AND FEMALE ORGANS OF GENERATION. 



in the vitelline membrane of man and the other vertebrate animals ; but 

 the globular form of the egg, the transparent and homogeneous texture 

 of the vitelline membrane, and the absence of any other material, of dif- 

 ferent refractive power, in the canal or orifice of the micropyle itself, 

 prevent its being detected in microscopic examination. 



Vitellus. The vitellus is a globular mass, of semifluid, tenacious con- 

 sistency, composed of a transparent and colorless albuminous material, 

 with oleaginous looking granules thickly disseminated throughout its 

 substance. Owing to the physical admixture of these two constituents, 

 it has a distinctly granular aspect, and a considerable degree of opacity. 

 Imbedded in the vitellus, at a point near its surface, and consequently 

 almost immediately beneath the vitelline membrane, is a clear, colorless, 

 transparent vesicle, of a rounded form, the germinative vesicle. In the 

 mammalian egg, this vesicle measures about .04 millimetre in diameter. 

 It presents upon its surface a nucleus-like spot, known by the name of 

 the germinative spot. Both the germinative vesicle and germinate spot 

 are partially concealed, in the uninjured condition of the egg, by the 

 granules of the surrounding vitellus. 



If the egg, while under the microscope, be ruptured by continued 

 pressure upon the covering glass, the semifluid vitellus is gradually 



expelled by the elasticity of the vitelline mem- 

 Fig-. 223. brane. It retains the granules imbedded in 

 its substance, but often allows the germina- 

 tive vesicle to become detached, and therefore 

 more distinctly visible. 



In man and the mammalians, the simple 

 form of egg above described, consisting mainly 

 of a vitellus of minute size, is sufficient for the 

 production of the embryo, since it is retained, 

 HUMAN OVUM, ruptured after fecundation, in the interior of the gene- 

 by pressure, showing the vi- ra tive passages, and absorbs the nutritious 



tellus partially expelled, the . t & .^ ' 



germinative vesicle, with it s materials for its subsequent growth from the 



t.ata and the tissues of the female parent. In the naked 



smooth fracture of the vitel- 

 line membrane. reptiles and in most fish, where the eggs are 



deposited and hatched in the water, the vitel- 

 lus is also of small size ; since the hatching takes place at a compara- 

 tively early period of development, and the requisite additional fluid 

 is supplied from the surrounding medium. But in birds, and in most 

 of the scaly reptiles, as serpents, turtles, and lizards, the eggs are de- 

 posited in a nest or in the ground, and there is consequently no external 

 source of nutrition for the support and growth of the embryo during 

 its development. In these instances the vitellus, or " yolk," is of large 

 size ; and the bulk of the egg is still further increased by the addition, 

 within the female generative passages, of layers of albumen and various 

 external fibrous and calcareous envelopes. The essential constituents 

 of the egg, nevertheless, still remain the same in character, and the 

 process of embryonic development follows its usual course. 



