270 



• I I'HALOPODA. 



yolk-sac and consequently lies ventrally. In Loligo, on the contrary, it is 

 found between the rudiments of the arms and the eyes. 



The position of the oral invagination in the neighbourhood of the former 

 aperture of the blastoderm might appear remarkable (Fig. 12G A), but may 

 possibly be accidental, being caused by the great reduction^of the yolk-sac. 

 We shall have to point out later that we are inclined to regard the slight 

 development of a yolk-sac in Grenacher's Cephalopod as a secondary 

 phenomenon. 



A. 



B. 



--■ch. 



Fig. 126.— Two stages of Grenacher's Cephalopod embryo. A, sei-n from the side 

 /;. from the ventral and postero-dorsal surface (after Grenacher). " r r/ :! , arms; an 

 rudiment of eve : ab, optic vesicle : ch, chromatophores ; ds, yolk-sac : htf, posterior 

 funnel-fold : k, rudiments of gill : m, mouth ; -ma, mantle : oe, aperture of the 

 blastoderm : ot, otocyst ; vtf. anterior funnel-fold : w, ciliated area. 



About the time when the rudiments of the arms appear, a large 

 swelling arises behind them on either side of the embryo, in connec- 

 tion with a depression, the rudiment of the eves (Fig. 126 A, au). 

 Soon after, the rudiment of the funnel appears in the form of two 

 pairs of folds. The anterior, slightly undulating folds are inclined 

 one to the other (Fig. 126 B). They unite later, in the way already 

 described for Loligo, to form the funnel (Fig. 127). The posterior 

 folds, which were originally distinct from the anterior though 

 running in the same longitudinal direction, fuse later with these latter, 

 and in any case yield the lateral portions of the funnel, that is, the 

 so-called nuchal muscles (Figs. 126 B and 127). At the points 

 where the anterior and posterior folds fuse, a process runs inward 



