'20 A CRITICAL PERIOD IN THE 



In the case of the horse, the yolk sac ceases to provide a suffi- 

 cient supply about the end of the seventh week ; but the embryo, 

 instead of being born at this period, has the new and more 

 efficient nutritive structures provided — the allantoic villi. 



During the first seven weeks the embryo is fixed to the lining 

 of the uterus by means of the embryonic sac. In the region of 

 the absorbing area some of the foetal cells blend with the adjacent 

 uterine cells. Around the absorbing area a circular adhesive 

 ring is formed. This ring shrinks as the absorbing area 

 diminishes. A supplementary grappling apparatus appears in 

 the form of a girdle, which becomes, up to the seventh week, 

 more and more complex, and travels towards the ring just men- 

 tioned as the allantois increases in size. 



The allantois, a simple breathing organ in the chick and young 

 reptile, is probably concerned in the aeration of the blood from 

 an early period — the third or fourth week — in the horse. But 

 by the end of the seventh week minute patches of enlarged cells 

 belonging to the outer tunic indicate that allantoic outgrowths 

 will soon appear, and by the end of the eighth week thousands of 

 villi have sprouted out from its surface, and are already lodged 

 in minute pits specially formed in the lining of the uterus. 

 These villi not only procure nourishment and fresh supplies of 

 oxygen, they further fix more or less firmly the embryonic sac to 

 the wall of the uterus. 



At the end of the third week of gestation, when the reproduc- 

 tive system passes through one of its periods of general excite- 

 ment, about one-fourth of the embryonic sac probably adheres to 

 the uterus ; but at the end of the sixth week, when another wave 

 of disturbance arrives, all the grappling structures are at one 

 pole. Hence there is probably more chance of the embryo 

 " slipping " at the end of the sixth than at the end of the third 

 week. About the end of the seventh week the supply of nourish- 

 ment by means of the yolk sac is coming to an end, and there 

 is perhaps still about this time a hereditary tendency for the 

 embryo to escape. Unless the new and more permanent nutri- 

 tive apparatus is provided, unless a countless number of villi 

 rapidly sprout out from the allantois, the embryo will die from 

 starvation during the eighth week, and in a few days be 

 discharged. 



