16 A CRITICAL PEEIOD IN THE 



admit of so much growth during the seventh week. 1 But while 

 there has been little change in the yolk sac, the allantoic sac 

 {all.) has now about three times the capacity it had at six weeks. 

 Whether the blood, as it circulates through the allantois, collects 

 nutritive material as well as fresh supplies of oxygen, I am not 

 yet able to say. The changes in the outer cells of the embryonic 

 sac are more interesting and suggestive than those in the yolk 

 sac and allantois. The ring (an.) around the absorbing area, 

 though flattened, still consists of delicate ridges. The girdle, still 

 circular at six weeks, has been folded so as to assume an extremely 

 irregular form (t.g., fig. 6). The outer margin of the girdle con- 

 sists of complex prominent ridges, which fit into corresponding 

 grooves in the uterine mucous membrane. These ridges extend 

 some distance along the main trunks of the vessels passing to and 

 from the allantois. Part of the space within the folded girdle is 

 occupied by numerous delicate ridges, which also doubtless help 

 in retaining the embryonic sac in its place. Up to this time the 

 embryo is suspended by means of the yolk sac from the upper 

 wall of the uterus. Hence all these ridges and processes fit into 

 grooves and depressions in the mucous membrane lining the roof 

 of one of the uterine horns. 2 



In the six-weeks' embryo the cells of the outer tunic beyond 

 the girdle are unaltered, but at the end of the seventh week this 

 tunic presents a countless number of minute dots. These clots (t.t.), 

 which are due to the elongation of small groups of cells, are the 

 first indication of the coming nutritive processes or villi. Later 

 a villus sprouts out in the position of each dot, the cells of the 

 tunic forming a covering for the allantoic outgrowth in very 

 much the same way as the finger of a glove covers a finger. 

 Whether these minute patches of elongated cells at the end of 

 the seventh week assist in taking in nourishment, and also in 

 fixing the embryonic sac, further investigations will doubtless 

 reveal. The interesting point to notice is that the rudiments of 



1 Although the absorbing area is small at the end of the seventh week, the 

 yolk sac contains countless numbers of minute granules, which are doubtless of 

 nutritive value. 



2 Usually the embryo, to start with, in the mare occupies the right horn, with 

 its head nearest the body of the uterus. Later, part of the embryo may lie in 

 the enlarged body of the uterus ; but the hind limbs remain to the last in the 

 right horn. 



