DEVELOPMENT OF SEPIA. 



273 



cleavage (see Fig. 109, and Fig. 110, of Loligo). The swellings 

 of the surface which at first are very vague and indistinct, become 

 gradually clearer and can soon be recognised as the rudiments 

 of the different organs, one of the first to grow distinct being a 

 circular depression at the centre of the germ-disc. This becomes 

 surrounded with a flat wall which is more or less pentagonal with 

 rounded corners (Fig. 128 A, sd, and ma). This represents the 

 rudiments of the shell-gland and the tnantlr, in the early appearance 

 of which Sepia resembles 

 the other Cephalopods 

 already described. 



Two broad prominences 

 which, at an earlier stage, 

 occupy almost half of the 

 germ-disc, may, following 

 KOLLIKEE, be described 

 a,s cepli alii' loin-.*. A large 

 pit appearing on each of 

 'them identifies them as 

 those highly developed 

 structures which, in other 

 ( "ephalopods, we regarded 

 as the optic swellings. 

 In Fig. 128 A, they are 

 represented at a some- 

 what later stage (kl). 

 The divergent character 

 of the development of 

 Sepia is specially shown 

 in these structures, which 

 here appear on an almost 

 flat surface ; elsewhere 

 they form two large and 

 very prominent swellings, 

 one on either side of the 



,'OUUL. 



a-. 



a f . 



FIG. 128. Germ-discs with young embryonic 

 rudiments of Sepia officinalis (after VIALLBTON 

 and KOLLIKBR). a, anus ; a r a s , the five pairs of 

 arms ; au, rudiments of eyes ; htf, posterior funnel- 

 fold ; k, gills ; kl, cephalic lobes ; ks, germ-disc ; 

 ///, oral aperture ; ma, mantle ; nk, nuchal cartil- 

 age ; ot, otocyst ; vtf, anterior funnel-fold. 



body. 



The germ-disc, on the side opposite to the optic swellings, is 

 bordered by a narrow band-like prominence which at first is almost 

 semicircular but soon extends round the greater part of the germ- 

 disc and then resembles an incompletely closed circular swelling, 

 This corresponds to the circular swelling which, in Loligo, runs round 



T 



