MOLLUSC A. 



237 



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time completely open to it ventrally, and only separated from the 

 actual yolk by the yolk membrane. There early grow out from the 

 walls of the mesenteron a pair of hepatic diverticula. 



As the mesenteric cavity extends it dilates at its distal extremity 

 into a chamber destined to form the stomach (fig. 127, mg). At 

 about this time the anus becomes perforated. Shortly afterwards 

 the mesenteron meets and 

 opens into the (esophagus 

 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 the yolk. 

 The yolk membrane through- 

 out the whole of this period 

 is quite passive, and has no 

 share in forming the walls 

 of the alimentary tract. 



The stomodcpMtn. The 

 stomodseum appears as an 

 epiblastic invagination at 

 the anterior side of the blas- 

 toderm, before any trace of 

 the mesenteron is present. 

 It rapidly grows deeper, and, 

 shortly after the mesenteric 

 cavity becomes formed, an 

 outgrowth arises from its 

 wall adjoining the yolk sack, 

 which gives rise to the sali- 

 vary glands (figs. 126 and 

 127, yls). Immediately be- 

 hind the opening of the sali- 

 vary glands there appears 

 on its floor a swelling which 

 becomes the odontophore, 

 and behind this a pocket of 

 the stomodreal wall forms 

 the sheath of the radula 

 (figs. 126 and 127 ; brd). Be- 

 hind this again the oeso- 

 phagus is continued dorsal- 

 wards as a very narrow tube, 

 which eventually opens into 

 the stomach (fig. 127). 



The terminal portion of the rudiment of the salivary gland divides 

 into two parts, each of which sends out numerous diverticula which con- 



FIG. 127. LONGITUDINAL SECTION THROUGH 



AN ADVANCED EMBRYO OF LOLIGO. (After Bo- 



bretzky. ) 



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 ; of,, oesophagus ; ?//. stomach ; 

 an. anus ; M. ink sack ; led. germinal tissue ; eih. 

 shell sack ; vc. vena cava ; g.vs. visceral gan- 

 glion ; g.pcl. pedal ganglion ; ac. auditory sack ; 

 tr. funnel. 



