12 A CRITICAL PERIOD IN THE 



nearly equatorial in position. The embryo, doubtless, still obtains 

 its nourishment by means of the yolk sac, but it may be assumed 

 the allantois is indirectly a fairly effective breathing organ. Com- 

 pared with the chick the development has been slow ; 1 but although 

 no great size has been reached, by the end of the fifth week the 

 foundations of the more important organs have been laid. Com- 

 pared both with earlier and somewhat later periods, the five-weeks' 

 embryonic sac is well fixed to the uterus ; for in addition to the 

 adhesions in the region of the absorbing area, and the hold ob- 

 tained by the plicated ring around this area, there is a very com- 

 plex and nearly equatorially placed girdle. As the embryonic sac 

 which contained .my five-weeks' embryo escaped from the uterus, 

 the girdle appeared as a whitish band nearly a quarter of an inch 

 in width. On section this girdle is seen to consist not merely of 

 elongated cells, as at the end of the fourth week, but of numerous 

 delicate folds separated from each other by deep furrows. That 

 it is concerned in fixing the embryo is evident from the adhesion 

 of uterine cells to its surface. 2 



When we take into consideration the size, structure, and posi- 

 tion of the girdle just described, and of the ring around the 

 absorbing area, it may, I think, be assumed that the horse embryo 

 is more firmly fixed to the uterus at the end of the fifth week 

 than at any subsequent period prior to the development of the 

 numerous processes (villi) which during the eighth week sprout 

 out like so many delicate rootlets from the outer surface of the 

 allantois. 



The Six- Weeks Horse Emory o. — Although the six-weeks' embryo 

 is little more than an eighth of an inch louger than the five-weeks' 

 one, 3 it is quite twice as heavy ; and though it appears to be 



1 There is not the same urgency to hurry on the development in the mammal 

 as in the chick — an embryo carefully hidden and guarded in a uterus is perfectly 

 safe. 



2 This girdle (not hitherto found in any mammal), in addition to fixing the 

 embryonic sac, may be the means of absorbing additional nourishment, which 

 may reach the embryo either through the yolk sac or the allantois. 



3 During the first seven weeks the amount of nourishment is likely to vary 

 considerably, and the extent of the absorbing area is also likely to vary. Hence 

 in collections of four or five or six weeks' embryos considerable differences in size 

 and extent of development would in all probability be met with. I should not be 

 surprised to find a six-weeks' embryo considerably larger than the one in my 

 possession, or a five-weeks' one somewhat smaller. Further, during the earlier 



