MOLLUSCA. 287 



The stomodaum. The stomodaeum appears as an epiblastic 

 imagination 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). Im- 

 mediately 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 stomodaeal 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 diver- 

 ticula which constitute the permanent glands. The greater part 

 of the original outgrowth remains as the unpaired duct of the 

 two glands 1 . 



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 con- 

 stricted off from the internal part of the sack, with which it remains con- 

 nected 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 cesopha- 



1 In Loligo only a single pair of salivary glands is present. 



