DEVELOPMENT OF THE HOUSE. 17 



the more permanent structures (the villi) concerned with the 

 nourishment of the embryo have appeared before the end of the 

 seventh week. Much must depend on what happens during the 

 beginning of the eighth week. If the embryo has been develop- 

 ing in a normal fashion, and is sufficiently vigorous, villi will 

 sprout out from every part of the now extensive allantoic sac, 

 and each villus as it grows will receive a covering of cells from 

 the outer tunic. If the lining of the uterus is in a healthy and 

 sufficiently vigorous condition, it will provide pits for the villi 

 as they develop. 



When the villi actually begin to grow is not yet known ; but 

 though small and simple, they are well formed, and present in 

 countless numbers at the end of the eighth week. Once estab- 

 lished, they increase in size and complexity (7 b , 7 C ), which im- 

 plies they become more and more capable of obtaining a plentiful 

 supply of nourishment, and of fixing the ever expanding sac con- 

 taining the embryo to the lining of the uterus. 



It is hardly necessary to explain that when an embryo is at 

 last provided with a countless number of allantoic villi, it is in 

 an infinitely better position than when its only means of obtain- • 

 ing supplies was a small porous area through which nutriment 

 filtered from the cavity of the uterus. Each villus may be com- 

 pared to a delicate branching rootlet, only it has the advantage (1) 

 of having blood rapidly circulating through its various branches, 

 by which the nutriment collected is conveyed at once to the 

 embryo, and (2) of having a relatively enormous amount of a 

 highly nutritive fluid — the maternal blood — brought sufficiently 

 near to admit of the materials required for building up the bones 

 and muscles and other parts of the embryo being readily absorbed. 

 But the villi may be said to play the part of leaves as well as 

 roots, for they admit of an exchange of gases. Through them the 

 foetal blood gets rid of its poisonous carbonic acid, and at the 

 same time secures a fresh supply of oxygen, without which tissues 

 can neither be built up nor maintained. When the villi begin 

 to act is not yet kuown. Though still very small, they are quite 

 visible to the naked eye at the end of the eighth week. If we 

 suppose they are fairly efficient by the middle of the eighth 

 week, it will be evident, if the seven-weeks' is compared with 

 the eight-weeks' embryo (fig, 7), that they are infinitely more 



