OF THE SKULL IN THE COMMON SNAKE. 
389 
description of the development of this capsule in the Chick, see Foster and Balfour., 
pp. 111-117.) 
The visceral folds and clefts can now be considered ; they are equally related to the 
sense-capsules and to the brain pouches or membrano-cranium. 
These descending visceral or haemal parts are represented in the body by a pair of 
laminae on each side, the somatopleure and the splanchnopleure (Foster and Balfour, 
pp. 62-67) ; but this division only exists above and in front when the visceral folds 
are first formed ; there is a cavity in each fold, but it soon closes. 
The pericardium (Plate 27, fig. 1, and Plate 29, fig. 1 , li. , pcd. ) is the foremost per- 
manent part of the cavity formed by division of the ventral lamina into two folds. 
Thus the haemal parts of the mouth and throat are formed in a generalized tract ; 
and, moreover, this does not grow into a wall, but (at first, at any rate) into a mere 
balustrade — columns and interspaces. 
Then as there are no structures in the body answering to the sense-capsules, and 
as the visceral folds are most intimately related to those special cephalic structures, we 
get three things in which the pleural elements of the head and throat difier from the 
costal elements of the body. 
A fourth difference might be mentioned, namely, that each rib arises from a distinct 
axial segment ; whereas the visceral growths of the head, even when they can be most 
clearly followed up to the axial parts, are downgrowths from a continuous structure. 
These folds with their interspaces (Plate 27, figs. 1 and 2 ; and see also in the Chick, 
‘Foster and Balfour,’ p. 118, fig. 37; p. 142, fig. 46 ; p. 180, fig. 56; and p. 181, 
fig. 57) form a regular series, some of which are in front of the mouth and some of 
them behind it. 
Of course those in front of the mouth are the most aberrant in form, the parts from 
which they grow being so much more specialized than the rootstock of the post-oral rods. 
At present there are only two post-auditory folds ; these answer to the gill-arches 
of the Ichthyopsida ; a third appears afterwards (Plate 27, fig. 3). These are the 
smallest of the series in the Snake. 
These folds are somewhat bulbous, so that the clefts are larger and more open above 
than further down ; their present development very partially walls-in the face and 
throat, and the pericardium lies between and below the three hinder pairs. 
The second post-oral or hyoid processes are larger than the branchial, and each has 
a large root above; this broad, fan-shaped “origin” embraces the whole lower face of 
the ear-sac. 
In front of this “root” we see the largest of the clefts, and, moreover, that the 
flexure of the head brings the bulbous distal ends of this fold, and the one in front of 
it, together. 
Thus this cleft becomes high in position as well as wide; it is called the “spiracle” 
in Sharks and Ganoids, and is well seen in the Selachians ; in adult Batrachians, and 
in the higher Yertebrata, it forms the tympano-eustachian passage. 
