MOLLUSC A. 



207 



The internal yolk-sack falls into three sections ; a cephalic section, 

 a section in the neck, and an abdominal section. Of these, that in 

 the neck is the first to be absorbed. The cephalic portion fills out 

 the ventral protuberance already spoken of. The hinder section 

 becomes occupied by the liver which exactly fits itself into this space 

 as it absorbs the material previously there. 



It will be convenient at this point to complete the account of the 

 Cephalopoda by a short history of their germinal layers, and by a 

 fuller description of the mantle, shell, and funnel than that given in 

 the preceding pages. 



It has already been shewn that in the region of the germinal 

 disc a thick layer of cells becomes interposed between the epiblast and 

 the yolk membrane. This layer (fig. 115 m) is mainly mesoblastic, 

 but also contains the elements which form the lining of the ali- 



c7is 



oe 



~brd 



FIG. 114. 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. oasophagus; ds. yolk-sack; chs. shell- 

 glaud; mt. mantle; pdh. mesenteron; x. epiblastic thickening between the folds 

 of the funnel. 



mentary tract. Its cells first become differentiated into mesoblast 

 and hypoblast after the shell-gland has become a fairly deep pit. 

 The mode of differentiation is shewn in fig. 114. On the posterior 

 side of the mantle, at the point marked in fig. Ill B, an, a cavity 

 is formed between the yolk membrane and the mesoblast cells (fig. 

 114, pdh}. This cavity is the commencement of the anal extremity 

 of the mesenteron, and the columnar cells lining it constitute the 

 hypoblast. The remainder of the lower layer cells are the mesoblast. 

 The mesenteron gradually extends itself till it meets the stomodseum 

 (fig. 127). The proctodaeum is formed as a shallow pit close to the 

 first formed part of the mesenteron. 



The mesoblast gives rise not only to the organs usually formed in 

 this layer, but also to the nervous centres, etc. 



The mantle and shell. The mantle first arises as a thickening 

 of the epiblast on the dorsal surface of the embryo. The thickened 

 integument, with the subjacent mesoblast, soon forms a definite pro- 

 jection, in the centre of which appears a circular pit (figs. 114 chs 



