336 



THE EVOLUTION OF MAN. 



anterior " intestinal gate ' (Fig. 102, at d). Just in the 

 same way the tail curves back against the ventral surface ; 

 the intestinal wall then encloses posteriorly a similar small 



r 3 



Ttt 



t 7i n 



vo I 3 i 



Fig. 101. — Longitudinal section through the embryo of a Chick (fifteenth 

 day of incubation). Embryo with arched dorsal surface (black) : d, intes- 

 tine ; o, mouth ; a, anus ; I, lungs ; h, liver ; g, mesentery ; v, auricle of 

 heart; k, ventricle of heart; h, arterial arches; t, aorta; c, yelk-sac; m, 

 yelk-duct ; w, allantois ; r, stalk of allantois ; n, amnion ; w t amnion- 

 cavity ; s, serous membrane. (After Baer.) 



cavity, the hind end of which is blind ; this is the pelvic 

 intestinal cavity. Its opening into the middle intestine 

 -"is the " hind intestinal gate." 



In consequence of these processes the embryo assumes a 

 form resembling a canoe lying bottom upward. Imagine a 

 canoe with rounded ends, and fitted with a little deck fore 

 and aft ; then turn this canoe upside down, so that its 

 arched bottom is uppermost: this affords an approximate 

 representation of this canoe-shaped embryo (Fig. 101,. e). 



