26 THE STOCK owner's ADVISER. 



the sow, sixteen or seventeen ; in the bitch, nine ; and in the rab- 

 bit, about fonr weeks; in the cat, eight weeks. The period of 

 gestation may vary to a remarkable extent in mares and cows. 

 This difference is owing to some extent to the difficulty of fixing 

 the exact time of conception. Male foals are carried two or three 

 days longer than female foals, and, as a rule, an old mare carries 

 her foal longer than a young mare. 



EMBRYOLOGY. 



The primary changes undergone by the fecundated ovum, 

 interesting as they are, can be merely alluded to here. By re- 

 peated fusion of the vitelline substance there arises a cellular 

 material called the embryogenic tissue, or blastoderm, out of 

 which the body of the embryo is formed. This tissue divides 

 into three layers — an outer, a middle, and an internal — called, 

 respectively, the epiblast, mesoblast, and hypoblast. From the 

 epiblast becomes developed the epidermis and its appendages 

 and the cerebro-spinal axis; from the hypoblast the mucous 

 membrane of the alimentary canal, while the mesoblast forms 

 the intervening organs and tissues. A linear indentation, the 

 primitive groove, appears on the epiblast; below and coincident 

 with this the notochord is formed, indicating the position of the 

 vertebral column. 



The chief appendages and coverings associated with fetal life 

 are the umbilical sack, the amnion, chorion, and allantois, the 

 placenta, and the umbilical cord. The outer covering of the 

 fetus is the chorion, a membrane derived from the blastoderm 

 and reflected over the fetus. The placenta is formed by the 

 connection of this covering with the mucous membrane of the 

 uterus. The inner covering is the amnion, which contains a 

 fluid, the liquor amnii. The allantois or urinary vesicle is re- 

 garded as taking its origin from the primitive intestine. Becom- 

 ing vesicular in form, to it proceeds the ducts of the Wolffian 

 bodies, or primordial kidneys. The umbilical sack proceeds 

 from the blastoderm. It covers the yelk, and like the allantois 



