33^ THE EVOLUTION OF MAN. 



wards extends into a long narrow canal, the yelk-duct. 

 Let us suppose we are within the cavity of the yelk-sac ; 

 we may then pass from it, through the yelk-duct (Fig. 101, m), 

 directly into the middle part of the intestinal canal, which 

 is still wide open. If from there we pass on into the 

 head portion of the embyro, we reach the head intes- 

 tinal cavity, the anterior end of which is blind. If, on the 

 other hand, we pass from the middle of the intestine back- 

 wards into the tail portion, we reach the pelvic intestinal 

 cavity, the hind end of which is blind (Fig. 94, g). The 

 first rudiment of the intestinal tube now consists, therefore, 

 strictly speaking, of three distinct sections : (1) the head 

 intestinal cavity, the hind end of which opens, through the 

 front intestinal gate, into the middle intestine ; (2) the 

 middle intestinal cavity which opens downwards, through 

 the yelk-duct, into the yelk-sac ; and (3) the pelvic intes- 

 tinal cavity, the front of which opens, through the posterior 

 intestinal gate, into the middle intestine. 



At first the mouth and anal openings are wanting. 

 The whole primitive intestinal cavity is entirely closed, and 

 is only connected in the middle by the yelk-duct with the 

 cavity of the intestinal germ-vesicle, which is also closed 

 (Fig. 94, g). The two future openings of the intestinal 

 canal, the anal opening and the mouth-opening, form only 

 secondarily, on the outside, and from the outer skin; 

 that is to say, a groove-like depression arises in the horn- 

 plate at the point where the mouth is afterwards situated, 

 and this grows deeper and deeper, growing towards the 

 blind front-end of the head intestinal cavity : this is the 

 mouth-groove. A similar groove-like depression appears 

 posteriorly on the outer skin, at the point where the anus 



