. THE DEVELOPMENT OF THE EMBRYO 



253 



membrana decidua, which is afterwards divided into three layers — the 

 decidua vera, decidua rejlexa, and decidua serotina; the last named is 

 especially devoted to the reception of the villi of the chorion. In the cavity 

 of tlie uterus a quantity of fluid rich in nucleated cells collects, in contact 

 with the deciduous membrane and the chorion, aiding in the process of 

 nutrition and purification of the foetal blood. 



The membranes which have been described as surrounding the embryo 

 also contain fluid, and the young animal during the whole of its existence 



Fig. 541. — Embryo of Horse at Seven Weeks 

 it. Embryo sac ; ally allz, allj, alh, allantoic ; am, amnion ; ijs^ yolk sac ; a-c, absorbing area 



in the uterus is surrounded and protected by water cushions of the most 

 perfect construction. 



It may be mentioned incidentally that all the membranes belonging to 

 the foetus, with a large portion of the deciduous linings of the uterus, are 

 cast off at the time of parturition as the after-birth, and the uterine mucous 

 membrane gradually returns to its former condition. 



Development of the Organs. — Up to this point the object of the 



writer has been to convey to the reader some idea of the very interesting 

 subject of embryonic development from the mature ovum, which is a mere 

 speck about the yifooth part of an inch in diameter, to the point at which 

 the rudiments of the young animal are formed, and the embryo is in va.scular 

 connection with the mother by the contact of tire vessels of the chorion 

 surrounding the ovum with these of the lining membrane of the uterus, so 

 arranged that nutriment may be tran.sferred from the parent to the offspring, 



Vol. III. 



82 



