CHAPTER II. 



FIG. 158. 



THE EGG, AND THE FEMALE ORGANS OF GENERATION 



THE egg, in man and mammalians, is a globular body, about 0.25 

 millimetre in diameter. It consists of an external closed sac, the 

 vitelline membrane, containing in its interior a spherical mass, the 

 vitellus. Of these two, the vitellus is the essential part of the egg, 

 since from its substance the rudiments of the embryo are formed. The 

 vitelline membrane is a protective envelope, serving to maintain the 

 form and integrity of the vitellus. 



Vitelline Membrane. The vitelline membrane is a smooth, transpa- 

 rent, colorless layer, about 0.01 millimetre in thickness. With a mag- 

 nifying power sufficiently moderate to in- 

 clude a view of the whole egg, it presents a 

 perfectly homogeneous aspect ; although with 

 higher powers, according to Klein, it exhibits 

 an appearance of vertical striation. Notwith- 

 standing its delicacy and transparency, it is 

 very elastic, and has a considerable degree 

 of resistance. If the mammalian egg be 

 placed under the microscope, surrounded by 

 fluid and covered with a thin glass, it may 

 be perceptibly flattened by pressure ; and 

 when the pressure is removed it resumes its 

 globular form. When slightly compressed in 

 this way, the apparent thickness of its vitel- 

 line membrane is increased, giving it the appearance of a rather wide, 

 pellucid border or zone, surrounding the granular and comparatively 

 opaque vitellus. From this circumstance it has received the name of 

 the " zona pellucida." 



In the vitelline membrane of many invertebrates, and also in that 

 of fishes, a minute opening has been discovered, termed the "micro- 

 pyle," leading into its cavity; and through this opening the spermatic 

 filaments of the male reach the vitellus. Such an opening may also exist 

 in the vitelline membrane of other vertebrate animals ; but the globular 

 form of the egg, the homogeneous texture of the vitelline membrane, 

 and the absence of any other material, of different refractive power, 

 in the orifice of the micropyle, are obstacles to its detection under the 

 microscope. 



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

 posed of transparent and colorless albuminous material, with oleaginous- 

 looking granules thickly disseminated throughout its substance. Owing 



584 



HUMAN OVUM, magnified 75 di- 

 ameters. a. Vitelliue mem- 

 brane. 6. Vitellus. c. Germi- 

 native vesicle, d. Germinative 

 spot. 



