250 



CEPHALOPODA. 



present. 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 pre- 

 viously 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 epi- 

 blast and the yolk membrane. This layer (fig. 115 m] is mainly 

 mesoblastic, but also contains the elements which form the 



c7is 



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. oesophagus ; ds. yolk-sack ; chs. 

 shell-gland ; nit. mantle ; pd/i. mesenteron ; jr. epiblastic thickening between the 

 folds of the funnel. 



lining of the alimentary tract. Its cells first become differenti- 

 ated 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. 1 1 1 B, an, a cavity is formed between the yolk 

 membrane and the mesoblast cells (fig. \\^,pdli). This cavity 

 is the commencement of the anal extremity of the mesenteron, 

 and the columnar cells lining it constitute the hypoblast. The 



