THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 117 
ive, the blood being supplied with all its needful constituents 
through the placenta by a much shorter process; indeed, the 
placental nutritive functions, so far as the foetus is concerned, 
may be compared with the removal of already digested material 
from the alimentary canal, though of course only in a general 
way. During fetal life the digestive glands are developing, 
and at the time of birth all the digestive juices are secreted in 
an efficient condition, though only relatively so, necessitating a 
special liquid food (milk) in a form in which all the constituents 
of a normal diet are provided, easy of digestion. 
Fia. 137.—Human germs or embryos from the second to the fifteenth week (natural size), seen 
from the left side, the arched back turned toward the right. (Principally after Ecker. 
Il, human embryo of 14 days; IL, of 3 weeks; JV, of 4 weeks; V, of 5 weeks; VI, of 
weeks ; VII, of 7 weeks ; VIII, of 8 weeks; XII, of 12 weeks; XV, of 15 weeks. 
Bile, inspissated and mixed with the dead and cast-off epi- 
thelium of the alimentary tract, is abundant in the intestine at 
birth in the human subject; but bile is to be regarded perhaps 
rather in the light of an excretion than as a digestive fluid. 
The skin and kidneys, though not functionless, are rendered 
unnecessary in great part by the fact that waste can be and is 
withdrawn by the placenta, which proves to be a nutritive, re- 
