382 A Manual of Veterinary Physiology. 



form, particularly in horses. They lose their awkwardness, 

 the outline of the frame becomes more consolidated and in 

 greater unison. In the male the neck becomes thick and 

 curved, the voice deepens, and the whole appearance 

 denotes life and vigour ; the temper is usually irritable and 

 uncertain, and often extremely vicious. 



The age at which procreation ceases is not known. 

 Fleming* says that mares have been known to produce 

 foals at 28, 32, and 38 years of age, and it is certain that 

 some of our best stallions are well advanced in years. 



Changes in the Ovum. — Before impregnation the germinal 

 vesicle and spot undergo changes, and the place of the 

 vesicle is taken by a spindle-shaped body ; at each end of 

 the spindle the elements of the yelk arrange themselves 

 in the form of a star. The peripheral pole of the spindle 

 projects outside the wall of the ovum, and is cut off; the 

 remaining pole is termed the female pro-nucleus. 



The ovum is impregnated in all probability in the 

 Fallopian tubes or tube. The spermatozoa passing through 

 the yelk envelope, or zona pellucida, and forming the male 

 pro-nucleus, which passes deeper into the ovum, and 

 approaching the female pro-nucleus, causes it to become 

 active. The change which now follows is segmentation of 

 the p-erminal vesicle, which divides and subdivides into an 

 indefinite number of small bodies. Next, the yelk in which 

 the small bodies of the germinal vesicle are distributed 

 begins to become segmented in twos, fours, eights, etc., 

 until the whole surface of the yelk is converted into 

 a mulberry-looking mass, consisting of minute spherules, 

 termed vitelline spheres. These now pass to the wall of 

 the vitelline membrane, and arrange themselves on its 

 inner surface, while the central portion of the ovum becomes 

 clear and transparent, The vitelline spheres which have 

 thus passed to the wall arrange themselves into a kind of 

 membrane termed the blastodermic or germinal membrane. 

 The cells of the germinal membrane accumulate at one 

 point and form an Opaque spot, the germinal area, which in 

 * ' A r eterin:uy Obstetrics.' 



