EGG AND FEMALE ORGAX3 OF GENEBATION. 



ON THE EGG, 



CHAPTER III. 



AND THE FEMALE 

 GENERATION. 



ORGANS OF 



Fig. 176. 



THE egg is a globular body which varies considerably in size in 

 different classes of animals, according to the peculiar conditions 

 under which its development is to take place. In the frog it mea- 

 sures T ' 5 of an inch in diameter, in the lamprey o 1 ^, in quadrupeds 

 and in the human species T .J . It consists, first, of a membranous 

 external sac or envelope, the vitelline membrane ; and secondly, of a 

 spherical mass inclosed in its interior, called the vitellus. 



The vitelline membrane in birds and reptiles is very thin, measur- 

 ing often not more than T ^<ytr f an i ncn in thickness, and is at the 



same time of a somewhat fibrous texture. 

 In man and the higher animals, on the 

 contrary, it is perfectly smooth, structure- 

 less and transparent, and is about T gVzr of 

 an inch in thickness. Notwithstanding 

 its delicate and transparent appearance, it 

 has a considerable degree of resistance 

 and elasticity. The egg of the human 

 subject, for example, may be perceptibly 

 flattened out under the microscope by 

 pressing with the point of a needle upon 

 the slip of glass which covers it; but it 

 still remains unbroken, and when the 



pressure is removed, readily resumes its globular form. When the 

 egg is somewhat flattened under the microscope in this way, by 

 pressure of the glass slip, the apparent thickness of the vitelline 

 membrane is increased, and it then appears (Fig. 176) as a rather 

 wide, colorless, and pellucid border or zone, surrounding the granu- 

 lar and opaque vitellus. Owing to this appearance, it has some- 

 times received the name of the " zona pellucida." The name of 



HUMAN OVUM, magnified 85 

 diameters, a. Vitelline membrane, 

 ft Vitellus. c. Germinative vesicle. 

 d. Germinative spot. 



