802 



ZOOLOGY 



SECT. 



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, 

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

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

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

 depressions, which subsequently develop into the statocysts. 



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

 more and more prominent. The eyes themselves are formed 



Z./Z. 



l']i.. 703. Two late stages in the development of Xioligo, seen from the funnel side. Letters as 



in preceding figures. (After Korschelt and Heider.) 



f'miii 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 

 tin' 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 

 b -comes very deep, separating it off from the rest of the egg ; the 

 latter, consisting of the greater part of the yolk enclosed in a thin 

 layer of blastoderm, forms a rounded appendage of the embryo the 



