DEVELOPMENT. 12t 



" sketched out," then become more distinct, and are seen to be 

 covered with a transparent shell. By this time the embryo is 

 able to move by its own mnscular contractions, and to swallow . 

 food ; it is therefore " hatched," or escapes from the egg. 



Gejiiholoijoda (xix, xx, 39-42). " In the dibranchiates the yolk 

 undergoes partial division, and the blastoderm (3'olk-sac) formed 

 upon the face of it b}^ the smaller blastomeres, spreads gradually 

 over the whole ovum, enclosing the larger and more slowly 

 dividing blastomeres. The mantle makes its appearance as an 

 elevated patch in the centre of the blastoderm, whilst the future 

 arms appear as symmetricall}^ disposed elevations of the per- 

 ipherj^, on each side of the mantle. Between these and the edge 

 of the mantle, two longitudinal ridges mark the rudiments of the 

 epipodia, while the mouth appears in the middle line in front of 

 the mantle, and the anus, with the rudiments of the gills, behind 

 it. The rest of the blastoderm forms the walls of a vitelline 

 sack, enclosing the larger blastomeres. 



" The pallial surface now gradually becomes more and more 

 convex, the posterior margin of the mantle growing into a free 

 fold , which encloses the pallial chamber and covers over the 

 gills. 



" The internal shell is developed in a sac formed by an invo- 

 lution of the ectoderm of the mantle. The epipodia unite behind, 

 and give rise to the funnel, while the antero-lateral portions of 

 the foot grow over the mouth, and thus gradually force the latter 

 to take up a position in the centre of the neural face, instead of 

 in front of it. The yolk-sack gradually diminishes, and the 

 contained blastomeres are finally taken into the interior of the 

 visceral sac, into which the alimentary canal is gradually drawn," 

 — Huxley, "Anatomy of Invertebrated Animals." 



At a later period of development, respiratory movements are 

 performed hy the alternate dilatation and contraction of the 

 mantle, and the position of the ink-bag is revealed by its color 

 through the transparent flesh. The shell of the young Sepia, 

 observes Kolliker, by the time it is prepared to leave the nidi- 

 mental capsule has already formed, but except the nucleus, 

 which is calcified, its fine layers are horny and transparent. 

 The fins are proportionally broader than those of the mature 

 animal.* 



The development of Nautilus, equally with its nidus is 

 unknown. 



Gastropoda (xx, 43-53 \ The transformation of the egg into 

 the embryo is preceded by .division of the yolk mass into blasto- 



* For a fuller account of cephalopodous development, see an excellent 

 paper on that of Loligo Pealei, by W. K. Bi-ooks, Anniversary Memoirs, 

 Boston Soc. Nat. Hist., 1880. 



