86 Animal Morphology. 



of the successors to their own species is the perfect mani- 

 festation of the tripartite membrane completed. 



But the ovum, impregnated by the granular matter of the 

 male, in process of time is not only protected in an air-tight 

 cavity, but soon a placenta is formed, and an investing serous 

 membrane, though materially modified ; holding within it 

 fluid for the better protection of the growing foetus, and also 

 to render pressure to the mother more equable and less 

 fixed. Here, then, we observe the ever-recurring fact, that 

 the serous membrane is always devoted to a mechanical 

 end, rather than that of an active vital function, as is 

 shown in the development of the amnion. 



True to its function as an excretory membrane, the 

 mucous membrane of the uterus, after due course of time, 

 in which the mucous and muscular membranes are under- 

 going a process of development and extension, in co- 

 relation to a more ultimate growth contained within, which, 

 by regular and equably sustained heat, aided by constant 

 nourishment, arrives at perfection in about 273 to 280 days. 

 A change now takes place, and the true excretory func- 

 tion of mucous membrane, aided by muscular force, begins 

 to manifest itself. For a few days there is a greater or less 

 amount of secretion or discharge, and a general lubrication, 

 to be followed by most violent and effectual efforts at 

 expulsion per vaginam. 



It is probable that the exciting cause of this great effort 

 arises primarily from a leaning in the mucous membrane 

 of the uterine system to assume a special function, which 

 is its legitimate function when fully developed. 



This new function produces a secretion of a stimu- 

 lating nature, and probably contains lime as one of its 

 essential elements, beginning at its free or Fallopian 

 end, and thence communicating stimulation to the 

 ovaries. 



