MOLLUSCA. 245 



In a young blastoderm of Sepia viewed from the dorsal 

 surface, a series of structures appear which are represented in 

 fig. in A. In the middle is a somewhat rhomboid prominence 

 which forms the rudiment of the mantle (mt). In its centre is a 

 pit which forms the shell-gland. On each side of the mantle is 

 a somewhat curved fold (/). These folds eventually coalesce to 

 form the funnel. They are divided into two parts by a small 

 body which forms the cartilage of the funnel. The smaller part 

 of the fold behind this body gives rise to the true funnel, the part 

 in front becomes (Kolliker) the strong muscle connecting the 

 funnel with the neck-cartilage. In front and to the sides are 

 two kidney-.shaped bodies (oc) the optic pits. Behind the mantle 

 are two buds (br} } the rudiments of the gills. 



FlG. in. TWO SURFACE VIEWS OF THE GERMINAL DISC OF SEPIA. 



(After Kolliker.) 



mt. mantle ; oc. eye ; /. folds of funnel ; br. branchiae ; an. posterior portion of 

 alimentary tract ; m. mouth, i, 2,3, 4, 5, arms ; /. cephalic lobe. 



In the somewhat later stage rudiments of the two posterior 

 pairs of arms make their appearance outside and behind the 

 rudiments of the funnel. The head is indicated by a pair of 

 lateral swellings on each side, the outer of which carries the eyes. 

 The whole embryo now becomes ciliated, though the ciliation 

 does not cause the usual rotation. At a slightly later stage the 

 second, third, and fourth pairs of arms make their appearance 

 slightly in front of those already present. The posterior parts of 

 the funnel rudiments approach each other, and the anterior meet 

 the rudiments of the neck-cartilage. The gills begin to be 

 covered by the mantle-edge, which now projects as a marked 

 fold. At a slightly later period two fresh rudiments may be 



