_The stomodæeum._ The stomodæum appears as an epiblastic invagination at the anterior side of the blastoderm, 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 salivary glands (figs. 126 and 127, _gls_). Immediately behind the opening of the salivary glands there appears on its floor a swelling which becomes the odontophore, and behind this a pocket of the stomodæal wall forms the sheath of the radula (figs. 126 and 127, _brd_). Behind this again the oesophagus is continued dorsalwards 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 constitute the permanent glands. The greater part of the original outgrowth remains as the unpaired duct of the two glands[117]. [117] In Loligo only a single pair of salivary glands is present. In the larva observed by Grenacher the anterior pair of salivary glands originated from independent lateral outgrowths of the floor of the mouth, close to the opening of the posterior salivary glands. _The yolk-sack of the Cephalopoda._ The yolk, as has already been stated, becomes at an early period completely enclosed in a membrane formed of flattened cells, which constitutes a definite yolk-sack. It is, in the more typical forms of Cephalopoda, divided into an external and an internal section, of which the former is probably a special differentiation of the median part of the foot of other cephalophorous Mollusca (_vide_ p. 272). At no period does the yolk-sack communicate with the alimentary tract. The two sections of the yolk-sack are at first not separated by a constriction. In the second half of embryonic life the condition of the yolk-sack undergoes considerable changes. The internal part grows greatly in size at the expense of the external, and the latter diminishes very rapidly and becomes constricted off from the internal part of the sack, with which it remains connected by a narrow vitelline duct. The internal yolk-sack becomes divided into three sections: a dilated section in the head, a narrow section in the neck, and an enormously developed portion in the mantle region. It is the latter part which mainly grows at the expense of the external yolk-sack. It gives off at its dorsal end two lobes, which pass round and embrace the lower part of the oesophagus. The passage of the yolk from the external to the internal yolk-sack is probably largely due to the contractions of the former.