THE PRINCIPLES OF LIFE. 83 



system has sufficiently matured to afford a surplus nutrition for 

 the reproduction of its kind, an increased supply of blood and 

 nervous energy to the ovary hastens the maturity of one or 

 more of these vesicles ; they burst as a ripened fruit must fall, 

 and the liberated ovum, descending the Fallopian tubes to the 

 womb, finds that the new-born activities of that organ have 

 elaborately prepared and fitted it as a home for its development 

 in the immediate future. 



In healthy females, from the approach of maturity to the de- 

 cline of life, when many other functions as well as the repro- 

 ductive ones are lost, this development and discharge of ova 

 persists, and with it the power of generation. Heat or rut is 

 the concomitant of such rupture and escape. 



But without the addition of the male or fertilizing element to 

 the ovum, its escape is but the prelude to its destruction, as it 

 no longer retains in itself the power of assimilation and in- 

 crease, but is thrown off, together with the exudation in the 

 womb, as a waste and useless thing. The male semen is an al- 

 buminous fluid secreted by the two testicles, and in health dis- 

 charged only during strong sexual excitement. It contains 

 myriads of minute organisms (spermatozoa), bearing a strong 

 resemblance in most animals when magnified, to tadpoles, and 

 having a similar but much more active power of motion. These 

 last elements appear to be the true fertilizing agents, as Spallan- 

 zani found that the fluid obtained by filtering the liquid had no 

 power to fecundate the ovum, whereas the material left on the 

 filter proved successful. Moreover, these are the only elements 

 in the male seminal fluid having the innate power of motion, 

 and since in animals killed a day or two after coition the ovum 

 is found in the Fallopian tube, undergoing that segmentation 

 and division of its yolk which results from impregnation, and is 

 besides surrounded by spermatozoa, there can be no reasonable 

 doubt that they alone are the fertilizing constituents. Further, 

 the spermatozoa are found in the testicles, the removal of which 

 destroys the power of procreation, and have not been found in 

 the semen of mules and other barren males. 



FORM OP THE OVUM AND ITS DEVELOPMENTAL CHANGES. 



As discharged from its ovarian (Graafian) vesicle, the uuim- 

 pregnated ovum is a globular mass, with an external granular 



