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), r). 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, r). 



The oesophagus and its annexes, viz. the radula, the salivary 

 glands, etc. (Fig. 290, (7), /, 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 (Argona.uta, 

 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 paripassu with the mantle (Fig. 290, t), 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), n), 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 



