THE TWO INTESTINAL CAVITIES. 335 



edges, but the ends also come together with the edges which 

 close, and form a navel. 



With this concentric closing of the intestinal tube is 

 connected the formation of two cavities, which are called 

 the head intestinal cavity and the pelvic intestinal cavity. 

 When the embryo gradually becomes detached from the 

 wall of the germ-vesicle, on which it at first lies flat, the 

 anterior and posterior ends are the first to be released, 

 while the central portion of the ventral surface continues 

 attached to the yelk-sac by the yelk-duct, or navel-duct 

 (Fig. 101, m). In the mean time the dorsal surface of the 

 body becomes much arched; the head end, on the other 

 hand, bends downward and against the breast, while the 

 tail end, in the same w^ay, presses against the abdomen ; 

 the embryo tries to roll itself together, as a hedgehog 

 makes itself into a ball to ward off its enemies. This arch- 

 ing of the back is caused by the quicker growth of the 

 dorsal surface, and is directly connected with the detach- 

 meut of the embryo from the yelk-sac (Fig. 101). In the 

 head there is no separation between the skin-fibrous layer 

 and the intestinal-fibrous layer, as is the case in the trunk, 

 but the two layers remain attached and form the so-called 

 " head-plates." Now as these head-plates free themselves , 

 at a very early period from the surface of the germ-area, 

 and grow, first downward toward the surface of the 

 intestinal germ-vesicle, and then backwards toward the 

 point, at which the latter passes into the intestinal groove ; 

 a small cavity is thus formed within the head portion, which 

 represents the foremost blind end of the intestine. This 

 is the small head intestinal cavity (Fig. 102, to the left 

 of d) ; its opening into the middle intestine is called the 



