33^ 



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 .7 



I 7i 7! vo J J 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 ; ?i, liver ; gr, mesentery ; -u, auricle of 

 heart; A-, ventricle of heart; fe, arterial arches; i, aorta; c, yelk-sac; w, 

 yelk-duct ; n^ allantois ; r, stalk of allautois ; w, amnion ; w, 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). 



