442 



MOLLUSCA 



FIG. 35(3. Embryonic development of Sepia 

 iifficinalis (after Kolliker). a, view of germinal 

 disc from above; the commencing embryo lying 

 on the yolk ; Br gills ; Tr folds of the funnel ; 

 Oc eye; M mantle, b, somewhat later stage, 

 from the front ; D yolk ; KV anterior, Kl" pos- 

 terior cephalic lobe ; mouth, c, later stage, 

 from the side ; 1-k first rudiments of arms, d, 

 older stage from the front ; 5 fifth pair of arms, 

 e, still later stage in lateral view ; the halves of 

 the funnel have united. 



anterior external pair bear the eyes 

 (Oc). On the outer edge of the disc 

 papilliform structures are formed, 

 the first rudiments of the arms. 

 In the later growth of this abso- 

 lutely symmetrical embryo, the 

 Cephalopod form becomes gradu- 

 ally more and more apparent : the 

 mantle projects considerably, and 

 grows over the gills and two parts 

 of the funnel, which fuse to form 

 the definite funnel. The cephalic 



FIG. 357. Almost 

 ripe embryo of 

 Sepia officinalis 

 from the dorsal 

 (anterior) face 

 (after Kolliker). 

 Ds yolk sac. 



lobes grow together between the 

 mouth and funnel, and on their 

 oral sides become more sharply 

 constricted off from the yolk, 

 which, with a few exceptions, 

 persists for some time as a yolk 

 sac. The yolk sac is attached 

 between the mouth and anus, and 

 may be regarded in the later 

 stages as a swollen up portion of 

 the foot. It should be noted that 

 the embryo is formed on the dorsal 

 side of the yolk sac, and never 

 completely surrounds it. 



