336 



THE EVOLUTION OF MAN. 



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

 same way the tail curves back against the ventral surface ; 

 the intestinal wall then encloses posteriorly a similar small 



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

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

 tine ; 0, mouth ; a, anus ; I, lungs ; h, liver ; g, mesentery ; r, auricle of 

 heart; /r, ventricle of heart; h, arterial arches; t, aorta; c, yelk-sac; to, 

 yelk-duct ; u, allantois ; r, stalk of allantois ; n, 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 fjate." 



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 tm-n this canoe upside down, so that its 

 arched bottom is iipj^ermost : this affords an approximate 

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



