THE CEPHALOPODA 
327 
also originate at the anal side of the blastoderm, behind the place 
where the shell gland is formed externally, and extend in the form 
of a crescent, right and left, between the ectoderm and endoderm, 
toward the anterior part of the blastoderm. After the mesoderm 
has been established in this manner, a thickened specialised portion 
of the endoderm constitutes the rudiment of the mesenteron and 
forms a little vesicle lying close upon and widely open to the 
yolk, and situated in the middle line, below the posterior part of 
the mantle, between the rudiments of the two branchiae (Fig. 
290, (6), 7). This vesicle is the rudiment of the alimentary tract : 
it ultimately gives rise to the stomach, the two lobes of the 
liver (which are separate from the first), and the intestine (Fig. 
290, 1). 
The oesophagus and its annexes, viz. the radula, the salivary 
glands, etc. (Fig. 290, (7), J, s), are formed by a precocious stomodaeal 
invagination, and the anus is formed later, by an excessively short 
proctodaeal invagination. Thus the mouth arises relatively near to 
the nutritive or vegetative pole, as it does in other Mollusca, and 
the less abundant the yolk, the nearer it is to the vegetative pole, 
as in the Cephalopod of Grenacher (Fig. 119, D). 
The mantle arises in the middle of the embryonic area (Fig. 257, 
pa), and in its centre is the shell gland, but the borders of the latter 
structure are reflected inwards and approach one another to form 
the shell sac. In certain highly differentiated Octopoda (Argonauta, 
Ray Lankester) the shell sac disappears before it is closed up, but 
in the Decapoda, with the exception of Spirula, it is completely 
closed, and it grows pari passu with the mantle (Fig. 290, ¢), while the 
shell develops within it. Posteriorly to the mantle, between it and 
the epipodium, appear the bud-like rudiments of the branchiae (Fig. 
290, (6), m), and the folds that form the branchial lamellae gradually 
make their appearance and become in their turn folded. As 
development advances the pallial cavity becomes deeper, and the 
branchiae are gradually covered by the mantle. 
Throughout the earlier part of embryonic life the cephalic mass is 
excessively large (Fig. 290, (8), (9)), but its preponderance insensibly 
diminishes in subsequent stages. This cephalic mass is formed by 
the antero-lateral regions of the embryonic area, and it bears the 
rudiment of an eye at each posterior corner (Fig. 290, (9), d). 
During these earlier phases of embryonic development the 
mouth is not in any sense surrounded by the circumoral append- 
ages. The foot, in fact, is at first formed by the lateral and 
posterior borders (Fig. 257, 1, 2, 3, 4, 5) of the embryonic area, 
and these borders are rapidly divided into ten projections in the 
Decapoda, or eight projections in the Octopoda and the Cephalopod 
of Grenacher (Fig. 119, D). But in the course of development 
these lobes, while they grow in length, also advance gradually 
