[FIG. 126. LONGITUDINAL VERTICAL SECTION THROUGH A LOLIGO OVUM WHEN THE MESENTERIC CAVITY IS JUST COMMENCING TO BE FORMED. (After Bobretzky.) _gls._ salivary gland; _brd._ sheath of radula; _oe._ oesophagus; _ds._ yolk-sack; _chs._ shell-gland; mt. mantle; _pdh._ mesenteron; _x._ epiblastic thickening between the folds of the funnel.] _The mesenteron._ The mesenteron is first visible from the surface as a small tubercle on the posterior side of the mantle between the rudiments of the two gills (fig. 111 B, _an_). Within this, as was first shewn by Lankester, a cavity appears. This cavity is as in Gasteropods open to the yolk-sack, and only separated from the yolk itself by the yolk membrane already spoken of. It is at first lined by indifferent cells of the lower layer of the blastoderm, which however soon become columnar and form a definite hypoblastic layer (fig. 126, _pdh_). Between the hypoblast and epiblast there is a very well marked layer of mesoblast. As the mesenteric cavity extends, its walls 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_). [FIG. 127. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF LOLIGO. (After Bobretzky.) _os._ mouth; _gls._ salivary gland; _brd._ sheath of radula; _ao._ anterior aorta; _ao1._ posterior aorta; _va._ branch of posterior aorta to shell sack; _ma._ branch of posterior aorta to mantle; _c._ aortic heart; _oe._ oesophagus; _mg._ 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.] The mesenteric cavity, still open to the yolk, gradually extends itself in a dorsal direction over the yolk-sack, but remains for some 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 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 the yolk. The yolk membrane throughout the whole of this period is quite passive, and has no share in forming the walls of the alimentary tract.