716 A MANUAL OF VETERINARY PHYSIOLOGY 



week. At the fifth week additional means of securing the embryo 

 to the wall are evident by an increase in the size and strength of 

 the original yolk-sac adhesion. There is also a girdle about J inch 

 wide, not hitherto found in any mammal (Ewart), placed around 

 the equator of the embryo (Fig. 251, t.g.). This girdle obtains 

 adhesion to the uterine wall, and so strengthens the original 

 anchorage. About the end of the sixth week the attachment of 

 embryo to uterus is again becoming precarious, for the yolk-sac 

 attachment area has become less (Fig. 252, a-c), while the girdle has 

 shifted from the equator to near the pole (Fig. 252, t.g.). At this 

 period Ewart considers the primitive ancestors of the horse were 

 born. 



At the end of the seventh week the supply of nourishment through 

 the medium of the yolk sac has nearly come to an end, the absorbing 

 area next the uterus is considerably reduced, and it is at this period 

 that an entirely new source of supply and attachment has to be 



Fig. 251. — Semi-diagrammatic Representation of a Four Weeks Horse 

 Embryo and its Foztal Appendages, Natural Size (Ewart). 



am., The amnion ; y.s., the yolk sac, which is vascular (v) as far as the circular 

 bloodvessel (s.t.), and crowded with granules which have entered by the 

 absorbing area (a, b, c) of the yolk placenta ; all., the allantois. The embryo 

 measures nearly § inch in length, and is curved so that the tail lies against 

 the head. 



found. The supply is furnished by means of the allantois, while 

 the additional attachment is furnished by the girdle becoming folded 

 into ridges, which fit into grooves and depressions in the mucous 

 membrane of the uterus. The outer cover of the embryo beyond 

 the girdle is dotted with numerous minute points, which subse- 

 quently become villi ; the villi are derived from a sprouting of the 

 allantoic sac, and as they grow are accommodated in pits in the 

 uterine wall. By the end of the eighth week this has been accom- 

 plished. The villi are not more than J inch long, even when full 

 grown, and at birth they are withdrawn from the uterine pits. Once 

 the villi have become established, the question of nourishment 

 becomes no longer a difficulty, and the critical stage in the develop- 

 ment of the horse is passed. 



