CHAPTER III. 



THE EGG, AND THE FEMALE ORGANS OF 

 GENERATION. 



THE egg, in man and in all species of mammalians, presents an essen- 

 tial similarity of form, size, and structure. It is a globular body, about 

 0.25 millimetre in diameter, and consists of two parts, namely, first, 

 an external closed sac, the vitelline membrane ; and, secondly, a sphe- 

 rical mass contained in its interior, the vitellus. Of these two, the 

 vitellus is the essential constituent part of the egg, since it is from its 

 substance that the rudiments of the embryo are formed. The vitelline 

 membrane is a protective envelope, destined 

 to maintain the form and integrity of the 

 vitellus. 



Vitelline Membrane. The vitelline mem- 

 brane is a smooth, transparent, colorless 

 layer, about .01 millimetre in thickness. 

 When viewed with magnifying powers suffi- 

 ciently moderate to include the view of the 

 whole egg, the membrane presents a perfectly 

 homogeneous aspect; although with higher 

 powers, according to Klein, it exhibits an HUMAN OVUM, magnified 75 



diameters. a, Vitelline mem- 



appearance of vertical striations. Notwith- brane. &. Vitellus. c. Germi- 

 standing its delicacy and transparency, it is j ve vesicle - *. Germinative 

 very elastic, and has a considerable degree 



of retistance. If the egg of the human species, or of any of the mam- 

 malians, be placed under the microscope, surrounded by fluid and 

 covered with a thin slip of glass, it may be perceptibly flattened out 

 by pressing upon the cover-glass with the point of a steel needle ; and 

 when the pressure is removed it readily resumes its globular form. 

 When the egg is partially flattened in this way, by the pressure of a 

 needle or by the weight of the cover-glass, the apparent thickness of the 

 vitelline membrane is increased, giving it the appearance of a rather 

 wide, pellucid border or zone, surrounding the granular and compara- 

 tively opaque vitellus. From this circumstance it has sometimes re- 

 ceived 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 " micropyle," 

 leading into the interior of the vitelline cavity ; and it is through this 

 opening that the filaments of the male seminal fluid penetrate, to reach 

 the vitellus. It is very possible that such an opening may also exist 



( 685) 



