184 



PHYSIOLOGY OF FARM ANIMALS 



[CH. 



tube and into the uterine cavity. Here they find a rich pabuhim 

 of nourishment provided by the prooestrous discharge. After a 

 few days each ovum sinks down into the uterine nnicous membrane 

 which in man grows round it forming the decidua reflexa. In 

 the mare up to the seventh week the young is nourished by 

 the yolk sac which opens widely into the embryonic gut and is 

 also the means of attachment of the young to the uterine wall. 

 The ovum meanwhile has undergone segmentation and become 

 converted into the embryo, and the embryonic or foetal mem- 

 branes grow outwards and form a bag containing a watery fluid. 

 The outer of these membranes is called the chorion and gives off 



cdl. 



Fig. 94. Diagrammatic representation of 

 4 weeks' lioi'se embryo and its foetal ap- 

 pendages (after Ewart from Smith, Messrs 

 Bailliere, Tindall and Cox), am. amnion, 

 y.s. yolk sac, sf. and v. blood vessels, 

 d. chorion, t.(j. area of special attach- 

 ment to uterine wall, all. allantoic 

 cavity, a.h.c. absorbing area of yolk sac. 



villi which project as finger-like processes into spaces in the 

 maternal tissue of the uterus. The attachment by the yolk sac 

 is then replaced by the attachment through the chorion. Mean- 

 while the uterus of the mother undergoes great changes, its 

 muscle walls becoming enormously developed, while the outer 

 portion of the connective tissue of the stroma gives rise to spindle- 

 shaped decidual cells and at the same time becomes very 

 greatly vascularised. This tissue forms the maternal placenta and 

 contains large blood sinuses into some of which the chorionic villi 

 project. In the meanwhile from a point within the embryo, 

 an outgrowth called the allantois arises, and the outer wall of 



