EGG AND FEMALE ORGANS OF GENERATION. 545 



vitelline membrane, however, is the one more generally adopted, 

 and is also the more appropriate of the two. 



The vitellus (b) is a globular, semi-solid mass, contained within 

 the vitelline membrane. It consists of a colorless albuminoid sub- 

 stance, with an abundance of minute molecules and oleaginous 

 granules scattered through it. These minute oleaginous masses 

 give to the vitellus a partially opaque and granular aspect under 

 the microscope. Imbedded in the vitellus, usually near its surface 

 and almost immediately beneath the vitelline membrane, there is a 

 clear, colorless, transparent vesicle (c) of a rounded form, known 

 as the germinative vesicle. In the egg of the human subject and of 

 the quadrupeds, this vesicle measures gj^ to 5 ^ of an inch in 

 diameter. It presents upon its surface a 

 dark spot, like a nucleus ((/), which is known ^ l * 177 ' 



by the name of the germinative spot. The 

 germinative vesicle, with its nucleus-like 

 spot, is often partially concealed by the 

 granules of the vitellus by which it is sur- 

 rounded, but it may always be discovered 

 by careful examination. 



If the egg be ruptured by excessive pres- HTMAX ov, ruptured by 

 sure under the microscope, the vitellus is P ressare ; 8howi the vitellu * 



partially expelled, the germiua- 



seen to have a gelatinous consistency. It tive vesicle at, and the *mooti, 

 is gradually expelled from the vitelline CM of the vitelline mPra - 



cavity, but still retains the granules and oil 



globules entangled in its substance. (Fig. 177.) The edges of the 

 fractured vitelline membrane, under these circumstances, present a 

 smooth and nearly straight outline, without any appearance of 

 laceration or of a fibrous structure. The membrane is, to all ap- 

 pearance, perfectly homogeneous. 



The most essential constituent of the egg is the vitellus. It is 

 from the vitellus that the body of the embryo will afterward be 

 formed, and the organs of the new individual developed. The 

 vitelline membrane is merely a protective inclosure, intended to 

 secure the vitellus from injury, and enable it to retain its figure 

 during the early periods of development. 



The egg, as above described, consists therefore of a simple 

 vitellus of minute size, and a vitelline membrane inclosing it. It 

 is such an egg which is found in the human subject, the quadru- 

 peds, most aquatic reptiles, very many fish, and some invertebrate 

 animals. In nearly all those species, in fact, where the fecundated 

 35 



