The pelagic Decapod larva described by Grenacher, which forms my second type, must be placed with reference to the development of the yolk-sack at the opposite pole to Sepia. Segmentation, as in other Cephalopods, is partial, but the blastoderm almost completely envelops the yolk before any organs are developed; and no external yolk-sack is present. At a stage slightly before the closure of the yolk-blastopore the mantle is formed as a slight prominence at the blastodermic pole of the egg, and even at this early stage is marked by the presence of chromatophores. The edge of the blastoderm is ciliated. At a slightly later stage the embryo becomes more cylindrical, the edge of the mantle becomes marked by a fold, which divides the embryo transversely into two unequal parts, a smaller region covered by the mantle, and a larger region beyond this. The yolk is still exposed, but rudiments of the optic pit and of two pairs of arms have appeared. The first-formed arms are apparently the anterior, and not, as in Sepia, the posterior.
At a still later stage, represented in lateral and posterior views in [fig. 113] A and B, considerable changes are effected. The yolk-blastopore is nearly though not quite closed. The mantle fold (mt) is much more prominent, and on the posterior side on a level with its edge may be seen the rudiments of the gills (br). The funnel is formed as two independent folds on each side (inf1 and inf2), which apparently correspond with the two divisions of the funnel rudiments in Sepia. The eye has undergone considerable changes. Close to each rudiment of the funnel may be seen a fresh sense organ—the auditory sack (ac). The ventral (upper in the figure) end of the body now forms a marked protuberance, probably equivalent to the foot of other Mollusca (vide p. [225]), at the sides of which are seen the rudiments of the arms (1, 2, 3). To the two previously present a third one, on the posterior side, has been added. The blastopore is placed on the anterior side of the ventral protuberance, and immediately dorsal to this is an invagination (os) which gives rise to the stomodæum. The ciliation at the edge of the blastopore still persists, but does not lead to the rotation of the embryo.
In later stages ([fig. 113] C) the blastopore becomes closed, and the mantle region increases in length as compared with the remainder of the body. The ventral halves of the funnel, each in the form of a half tube, coalesce together to form a single tube (inf) in the same manner as in Sepia. A shallow proctodæum (an) is formed between the two branchiæ. The eyes (oc) stand out as lateral projections, and the arms become much longer.
Fig. 113. Three Embryos of a Cephalopod with a very small yolk-sack.
(After Grenacher.)
a. blastopore; br. branchiæ; inf.1 and inf.2 posterior and anterior folds of the funnel; g.op. optic ganglion (?); oc. eye; wk. white body; ac. auditory pit; os. stomodæum; an. anus; mt. mantle; 1, 2, 3. 1st, 2nd, and 3rd pairs of arms.
Still later a fourth pair of arms is added as a bud from each of the posterior pair, and with the growth in length of the arms the suckers make their appearance. The mouth is gradually carried up so as to be surrounded by the arms. The ciliation of the surface becomes more extensive.
During the whole of the above development the interior of the embryo is filled with yolk, although no external yolk-sack is present. The internal yolk-sack falls into three sections; a cephalic section, a section in the neck, and an abdominal section. Of these, that in the neck is the first to be absorbed. The cephalic portion fills out the ventral protuberance already spoken of. The hinder section becomes occupied by the liver which exactly fits itself into this space as it absorbs the material previously there.
It will be convenient at this point to complete the account of the Cephalopoda by a short history of their germinal layers, and by a fuller description of the mantle, shell, and funnel than that given in the preceding pages.