746 



ZOOLOGY 



SECT. 



general level of the blastoderm ; this is the rudiment of the 

 mantle. On the surface of this is developed a depression which 

 subsequently forms a closed sac the shell-gland (Fig. 659, sh. gl.}. 

 Below the mantle i.e. nearer the vegetal pole appear two eleva- 

 tions each with a pit-like depression, the rudiments of the eyes ; 

 and still nearer the vegetal pole a series of paired elevations, 

 the rudiments of the arms. 



After the complete enclosure of the yolk by the blastoderm, the 

 mouth (mo.} is developed as an oval depression between the rudi- 

 ments of the eyes. Immediately in front of the edge of the mantle 

 appear two short ridges, the beginnings of the gills (cten.}, and a pair 

 of folds the posterior funnel folds (post. f. f. ) which are formed 

 between these and the eyes are the first rudiments of the funnel ; 



B 



z. 



-bl 



yk 



CC^/\^>-iCC c7 



yk.ep 



FIG. 65S. Sections through the edge of the blastoderm of Sepia at three successive stages ; 

 Itl. blastoderm; '//.. yolk; ak. cj>. yolk epithelium. (From Korschelt and Heider, after 

 Vialleton.) 



the greater part of which, however, is formed from a second pair 

 of folds- -the anterior funnel folds (n/tt.f.f.) developed further 

 forwards. Behind the anterior funnel folds appear two pit-like 

 depressions, which subsequent^ develop into the otocysts. 



The elevations on which the eyes (eye} are situated become 

 more and more prominent. The eyes themselves are formed 

 from a part only of these elevations : each is a pit which sub- 

 sequently becomes closed to form a vesicle the optic vesicle : 

 later an ingrowth of the ectoderm over this gives rise to 

 the lens. 



The embryo covers only a part of the egg, and as it develops, it 

 withdraws itself more towards the animal pole, at which the 

 germinal disc was originally situated, a constriction, which soon 

 becomes very deep, separating it off from the rest of the egg ; the 



