Fig. 114. Longitudinal vertical section through a Loligo ovum when the mesenteric cavity is just commencing to be formed.
(After Bobretzky.)
gls. salivary gland; brd. sheath of radula; oe. œsophagus; ds. yolk-sack; chs. shell-gland; mt. mantle; pdh. mesenteron; x. epiblastic thickening between the folds of the funnel.
It has already been shewn that in the region of the germinal disc a thick layer of cells becomes interposed between the epiblast and the yolk membrane. This layer ([fig. 115] m) is mainly mesoblastic, but also contains the elements which form the lining of the alimentary tract. Its cells first become differentiated into mesoblast and hypoblast after the shell-gland has become a fairly deep pit. The mode of differentiation is shewn in [fig. 114]. On the posterior side of the mantle, at the point marked in [fig. 111] B, an, a cavity is formed between the yolk membrane and the mesoblast cells ([fig. 114], pdh). This cavity is the commencement of the anal extremity of the mesenteron, and the columnar cells lining it constitute the hypoblast. The remainder of the lower layer cells are the mesoblast. The mesenteron gradually extends itself till it meets the stomodæum ([fig. 127]). The proctodæum is formed as a shallow pit close to the first formed part of the mesenteron.
The mesoblast gives rise not only to the organs usually formed in this layer, but also to the nervous centres, etc.
The mantle and shell. The mantle first arises as a thickening of the epiblast on the dorsal surface of the embryo. The thickened integument, with the subjacent mesoblast, soon forms a definite projection, in the centre of which appears a circular pit ([figs. 114] chs and [115] shs). This pit, which has already been spoken of as the shell-gland, resembles very closely the shell-gland of other Mollusca. The fold around the edge of the shell-gland grows inwards so as gradually to circumscribe its opening, which before long becomes completely obliterated; and the gland forms a closed sack lined by epiblast which grows in an anterior direction ([figs. 114] and [127] cch).
Fig. 115. Diagram of a vertical section through the mantle region of an embryo Loligo. (From Lankester.)
[This figure is turned the reverse way up to fig. 114.]