THE EARLY DEVELOPMENT OF THE MAMMAL 409 



reaches its maximum during the second month. The intestine 

 is rapidly withdrawn later and at about nine or ten weeks is 

 completely retracted into the embryonic body cavity. Beyond 

 this expanded region the umbilical cord is considerably elon- 

 gated and begins to show its characteristic spiral twisting. The 

 yolk stalk is correspondingly elongated and now loses its 

 endodermal cavity. 



FIG. 169. Human embryo of 22.8 mm. (fifty-three days). After Minot, 



X4. For description see text. 



The limbs grow rapidly during this month; they become dif- 

 ferentiated into their three chief regions, and in the hand and 

 foot the digits become clearly differentiated (Figs. 168, 169). 

 By the end of this month the limbs project beyond the outlines 

 of the body. The tail gradually recedes and by the time the 

 embryo is two months old is scarcely visible externally. The 

 structures of the facial region develop rapidly during this 



