Fig. 111 B represents a view from the dorsal surface of an embryo at this stage. In the centre is the mantle with the shell-gland which is now very considerably raised beyond the general surface. Concentric with the edge of the mantle are the two halves of the funnel, the anterior half meeting the dorsal or neck-cartilage and the posterior halves approaching each other. The oral invagination is shewn at _m_ and the anal immediately in front of _an_. The gills, nearly covered by the mantle, are seen at _br_. At _p_ are the cephalic swellings, and the eye is seen at _oc_. The arms 1-5 form a ring outside these parts. The whole of the embryo, with the exception of the gills, the funnel, and the outer border of the blastoderm, is richly ciliated. The embryo up to this time has had the form of a disc or saucer on the surface of the yolk. After this stage it rapidly assumes its permanent dome-like form, and becomes at the same time folded off from the yolk. The blastoderm is very slow in enveloping the yolk, and the whole yolk is not completely invested till a considerably later stage than that represented in fig. 111 B. As soon as the blastoderm covers the yolk-sack cilia appear upon it. The mantle grows very rapidly, and its free border soon projects over the funnel and gills. After the two halves of the funnel have coalesced into a tube, it comes to project again beyond the edge of the mantle. On the completion of the above changes the resemblance of the embryo to a Cuttle-fish becomes quite obvious. Three of the stages in the accomplishment of these changes are represented in fig. 112. [FIG. 112. SIDE VIEWS OF THREE LATE STAGES IN THE DEVELOPMENT OF SEPIA. (After Kölliker.) _m._ mouth; _yk._ yolk-sack; _oc._ eye; _mt._ mantle.] To the ventral side of the embryo is attached the enormous external yolk-sack (_yk_), which is continuous with an internal section situated within the body of the embryo. The general relations of the embryo to the yolk will best be understood by reference to the longitudinal section of Loligo, fig. 127. The arms gradually increase in length, and the second pair passes in front of the first so as eventually to lie completely in front of the mouth. The arms thus come to form a complete ring surrounding the mouth, of which the original second pair, and not, as might be anticipated, the first, completes the circle in front. The second pair develops into the long arms of the adult. After the embryo has attained more or less completely its definite form (fig. 112 C) it grows rapidly in size as compared with the yolk-sack. The latter structure is at first four or five times as big as the embryo, but, by the time of hatching, the embryo is two to three times as big as the yolk-sack. Loligo mainly differs from Sepia in the early enclosure of the yolk by the blastoderm, and in the embryo exhibiting the phenomena of rotation within the egg-capsule so characteristic of other Mollusca.