THE FEMALE ORGANS OF GENERATION. 585 



to the admixture of these two constituents, it has a granular aspect, 

 and a considerable degree of opacity. Imbedded in the vitellus, near 

 its surface, and consequently almost immediately beneath the vitelline 

 membrane, is a clear, colorless, transparent, rounded sac the germina- 

 tive vesicle. In the mammalian egg, this vesicle measures about .04 

 millimetre in diameter. It presents upon its surface a nucleus-like spot, 

 known as the germinative spot. Both the germinatiye vesicle and the 

 germinative 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 pressure on 

 the cover-glass, the vitellus is gradually expelled by the elasticity of 

 the vitelline membrane. It retains the gran- 

 ules imbedded in its substance, but the germi- FlG - 1 59 - 

 native vesicle often becomes detached, and 

 therefore more distinctly visible. 



In man and mammalians, the simple form of 

 egg above described is sufficient for the pro- 

 duction of the embryo, since it is retained, after 

 fecundation, within the generative passages, 

 and there absorbs the nutritious materials for 

 its subsequent growth. In the naked reptiles HUMAN OVUM, ruptured by 

 and in most fish, where the eggs are deposited pressure, showing the vitellus 



. , , . ! w - 1 f part ially expelled, the genni- 



and hatched in water, the vitellus is also of native vesicle, with its germi- 

 small size: since the hatching takes place at native ^ at a < and th , : 



smooth fracture of the vitel- 



an early period of development, and the req- ime membrane, 

 uisite additional fluid is supplied from the 



surrounding medium. But in birds and most of the scaly reptiles, 

 as serpents, turtles, and lizards, the eggs are deposited in a nest or in 

 the ground, with no external source of nutrition for the embryo. In 

 these instances the vitellus, or "yolk," is of large size; and the bulk 

 of the egg is further increased by the addition, within the genera- 

 tive passages, of albuminous material and fibrous or calcareous en- 

 velopes. 



Ovaries and Oviducts. The eggs are produced in the interior of 

 certain organs, situated in the abdominal cavity, called the ovaries. 

 These organs consist of a mass of vascular connective tissue, inclosing 

 numerous globular sacs or follicles, the " Graafian follicles ;" so called 

 from the anatomist * who first fully described them as constituent parts 

 of the ovary. Each Graafian follicle contains an egg, which varies 

 more or less in size and appearance in different classes of animals, but 

 which has always the same essential characters, and is produced in the 

 same way. 



The egg thus grows in the ovarian sac, like a tooth in its follicle ; 

 and forms, accordingly, a constituent part of the body of the female. 

 It is subsequently separated from its attachments, and thrown off; but 



* Regner de Graaf, Opera Orania. Amstelscdami, 1705, p. 228. 



