3° Journal of Agriculliire. [ii Jan., 1909. 



ovum. Pregnancy begins and the sexual cvcles are intermitted. But 

 should no such ovum arri\e the uterus returns to its normal state in a 

 period of metcestrum. Then follows a resting stage of anoestrum or 

 dicestrum and then the cycle begins afresh. Not only pregnancy but 

 lactation (nursing) may check the occurrence of these cycles. This rule, 

 ho\ve\er, is not without exceptions for man\- rodents may suckle a litter 

 of young while pregnant with another litter, and the mare generally 

 experiences oestrus nine days after giving birth to a foal. 



PREGNANCY. 



The numl)er of fertilized ova that reaches the prepared uterus at one time 

 varies with the species of animal. In the larger mammals one onlv is the 

 rule. In the cat three to six, in the dog four to ten, and in the pig six 

 to twelve ova are fertilized at the same time. In the bird and lower animals 

 the fextilized ova leave the body with a supply of nourishment and continue 

 their development outside the mother. In mammals however the ova 

 depend from the first for nourishment on the maternal uterus. When the 

 small mass of cells formed from a fertilized mammalian ovum reaches the 

 uterus at the proper stage in the sexual cycle it remains adherent to the 

 uterine wall. Subdivision and growth of the cells occur rapidlv. After 

 a short time a grouping of cells is evident, and the beginnings of skin, gut, 

 and central nervous s}Stem become apparent. So far the nourishment 

 required has been derived by simple absorption from the secretion of the 

 uterine lining membrane, but when the embr\o develops a system of blood 

 vessels and a heart a more efficient method of obtaining nourishment is 

 required. The em.bryo encloses itself in a bag containing a watery fluid 

 and compo.sed of a fairly tough membrane. The wall of the bag fits itself 

 to the wall of the uterine cavity. The chief use for this is to shield the 

 delicate embryo from injury and also to bear the brunt of the work of 

 dilating the neck of the uterus and the vagina when the young is born. 

 From a point in the embryo, which is afterwards recognised as the navel, 

 a curious outgrowth arises called the allantois, which eventually blends 

 with a portion of the wall of the membranous bag to form the placenta. 

 The placenta sends long finger-shaped processes into similarly shaped de- 

 pressions in the uterus which are lined bv walls richly supplied with blood 

 vessels. Two arteries from the embryo pass along the stalk of the allantois 

 and break up into a dense meshwork of capillaries in the placenta ; then the 

 blood is collected into venules and finally into one large vein which runs 

 parallel with, and close beside, the two arteries mentioned and brings back 

 purified and enriched blood to the embryo. It must be remembered that 

 there is no direct connexion between the blood of the foetus and that of 

 the mother. The embrvonic heart pumps blood tl^icii'^h its own 

 todv and through the placenta. In the finger-shaped jwocesses or 

 villi of the placenta the .separation from the maternal blood is 

 effected bv a verv thin membrane through which the exchange 

 takes place. The blood of the foetus takes up oxygen from, and 

 gives off carbon dioxide to. the maternal blood. Tt was formerly thought 

 that the nutriment required t)v the embryo simi)ly filtered throi'gh from 

 the uterine blood vessels into the capillaries of the placental villi, but there 

 is reason to believe that the placenta is a tligestive organ breaking down 

 the necessary constituents of the maternal blood pnd opssing them into the 

 blood of the foetus. The stalk of the allantois with its vein and two 

 arteries is known in the later stages of embr\onic development as the 



