DEVELOPMENT OF THE EMBRYONIC RUDIMENT LOLIGO. 



253 



cell-layers ; this corresponds to the highest point of the dorsal side, 

 as may be seen by a comparison with the later embryonic stages. 

 This swelling, which is the rudiment of the mantle, soon becomes 

 large, and, in the Loligo examined by Brooks, rests like a cap upon 

 the embryo (Fig. 115 A), but, in Loligo vulgaris, is less distinctly 

 marked at this stage. At the stage depicted in Fig. 115 A, the 

 greater part of the circular margin of this cap-like swelling has risen 

 up in the form of a fold from the embryonic body (Figs. 116 and 117, 

 ma), the mantle thus forming in the same way as in the other 

 Mollusca. As already mentioned, the swelling lies on the dorsal side 

 and a structure appears here which is comparable with the shell-gland 

 of other Molluscs. This is an ectodermal depression (Fig. 115 B, 

 and 116 A, sv/) which at first is shallow but deepens later, and does 



R 



B 



sd 



an 



s ea- 



rn 



Sv 



\ 



■ i 



I 



d r 



Fig. 115. — Two early stages in the development of Loligo PecUii (after Brooks), ar, 

 rudiment of arm ; au, rudiment of eye ; tl, yolk ; m, rudiment of mantle; r, the 

 ciliated margin of the blastoderm ; sd, shell-gland. 



not flatten out again as in other Molluscs (pp. 34, 92, 126), but 

 becomes a large pouch (Fig. 132, sd, p. 282). This peculiarity is 

 connected with the fact that Loligo possesses an internal shell.* In 

 those land-Pulmonates that are provided with an internal shell, we 

 also found (p. 187) that such a shell-pouch formed from the shell-gland. 

 There is a certain similarity between the swelling which becomes 

 differentiated into the rudiment of the mantle and the shell-gland 

 and the first indications of the eyes which also arise in the form of 

 swellings each carrying a pit-like depression (Fig. 115 B). They lie 

 at the two sides of the body below the mantle (Fig. 116 B, au). 



* ('/. p. 287, on the significance of the internal shell of the Cephalopoda. 



