286 



ALIMENTARY TRACT. 



vrr, 



, f/la 



meet the epiblast, and at the point of contact of the two layers 

 the epiblast becomes slightly pitted in. At this point the anus 

 is formed at a considerably later period (fig. 127, an}. 



On the ventral side of the primitive mesenteron an outgrowth 

 appears very early, which becomes the ink sack (fig. 127, bi). 



The mesenteric cavity, still open to the yolk, gradually ex- 

 tends itself in a dorsal di- 

 rection over the yolk-sack, 

 but remains for some time 

 completely open to it ven- 

 trally, and only separated 

 from the actual yolk by 

 theyolk membrane. There 

 early grow out from the 

 walls of the mesenteron 

 a pair of hepatic diver- 

 ticula. 



As the mesenteric 

 cavity extends it dilates 

 at its distal extremity 

 into a chamber destined 

 to form the stomach (fig. 

 127, ing). At about this 

 time the anus becomes 

 perforated. Shortly af- 

 terwards the mesenteron 

 meets and opens into 

 the oesophagus at the 

 dorsal extremity of the 

 yolk sack, but at the time 

 when this takes place the 

 hypoblast has extended 

 round the entire cavity, 

 and has shut it off from 



-tr 



ypd 



FIG. 127. LONGITUDINAL SECTION THROUGH 



AN ADVANCED EMBRYO OF LOLIGO. (After Bo- 



bretzky.) 



the yolk. The yolk mem- 

 brane throughout the 

 whole of this period is 



os. mouth ; gls. salivary gland ; brd. sheath of 

 radula ; ao. anterior aorta; ao l . posterior aorta; 

 va. branch of posterior aorta to shell sack ; ma. 

 branch of posterior aorta to mantle ; c. aortic 

 heart ; oe. oesophagus ; nig. stomach ; an. anus ; 

 bi. ink sack ; kd. germinal tissue ; eih. shell sack ; 

 vc. vena cava ; g.vs. visceral ganglion ; g.pd. pedal 

 ganglion ; ac. auditory sack ; tr. funnel. 



quite passive, and has no 



share in forming the walls of the alimentary tract. 



