266 



COMPARATIVE ANATOMY 



CHAP. 



and has a free mantle edge all round its base. The gills have shifted further 

 into the mantle cavity, which is now larger, and lies posteriorly. The rudiments 

 of the funnel also now lie close to the mantle, and are so approximated pos- 

 teriorly as nearly to touch. The rudiments of the arms have shifted from 

 behind further forward round the rudiments of the head. As the whole embryo 

 projects more distinctly from the yolk, the rudiments of the arms shift nearer to 



FIG. 221. Ontogeny of Sepia (after Koelliker). A-E, Five stages of development. The free 

 surface of the germinal disc which lies on the yolk is seen, its centre corresponding with the dorsal 

 point of the visceral dome of the adult Sepia. The anterior side of the embryo lies lowest in the 

 figures, a, Visceral dome with mantle ; b, rudiment of eye ; c, rudiment of gill ; d, halves of the 

 funnel ; e, rudiment of the funnel cartilage belonging to the apparatus for closing the mantle ; 

 /, peripheral part of the blastoderm, which, growing all round the yolk, forms the yolk-sac ; 

 g, mouth ; It,, posterior cephalic lobe ; i, anterior cephalic lobe ; A', anus ; 5, anterior or first pair 

 of arms ; 4, 3, 2, 1, second, third, fourth, and posterior pairs of arms. 



one another and under the rudiments of the head. The anus is already covered by 

 the mantle fold. 



5th Stage (Figs. 221 E and 222 B, H). The arms shift still nearer to one 

 another (i.e. towards the axis of the embryo), grouping below the rudiments of the 

 head (which have become fused), and form a somewhat narrow circle on the ventral 

 side in such a way that, when the embryo is seen from the dorsal side, some of them 

 are hidden by the head. As a consequence of this the embryo, which is already 

 recognisable as a young Sepia, now becomes sharply constricted from the yolk 

 beneath it. The free edges of the rudiments of the funnel fuse and move to a position 

 within the mantle cavity. 



6th Stage (Fig. 222 C). The rudiments of the head and arms have now 

 assumed the typical position to form the "head" (Kopffuss). The embryo is now 

 altogether distinct from the yolk, to which it merely hangs instead of, as before, 

 lying upon it. The blastoderm finally grows round the yolk and so forms a yolk 

 sac. At first this sac is four or five times the size of the embryo, but in proportion 



