392 
MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
Thus the highly-specialized parts round the oral clefts get an additional nerve- 
supply from the 7th ; and the 5th gives off' its first branch to the front of the face, 
which has no other chance of a supply of this kind, the neural outgrowths of the fore 
brain being specialized to the utmost for the 1st and 2nd sense-capsules, and three of 
the motor nerves going to the orbital muscles. 
Second Stage. Embryo Snakes, about 1 inch in length. 
This stage (see Plate 27, figs. 3-6, and Plate 29, fig. 2) gives us some important 
characters in the fast modifying head, which is now one-third larger than in the last 
stage. 
The bend of the head is fully as great as it was (figs. 1 and 3) ; the mid brain (C 3 ) 
projects still more ; and the hemispheres are now fairly divided (fig. 4, C lfl ). 
The brain is much more solid and complex (Plate 29, fig. 2) ; the space between the 
mid and hind brain (middle trabeculae, m.tr.) is much less ; the pituitary body is much 
larger, but is quite distinct from the infundibulum (pi/., inf.). 
The notochord (Plate 29, fig. 2, nc.) is straight and has receded from the open, oval, 
large pituitary space (Plate 27, fig. 6,py.); the roof of the head, over the long hind 
brain (figs. 3 and 5, C 3 ) is very thin, and the cerebral cavity is still large (Plate 29, 
fig. 2, C 3 ). 
The large splanchnic pouch (pericardium) has retreated ; its end was opposite the 
first post-oral cleft (fig. 2) ; it is now opposite the middle of the third post-oral fold 
(fig. 4). It is evident that the visceral folds are above and in front of the permanent 
division of the body wall into “ somatopleure ” and “ splanchnopleure.” 
The oral cavity (rn.) is now a large square space, with extended angles that open out 
between the maxillary and mandibular rudiments (fig. 4, m.xp., mn). 
A comparison of the side views shows how this has taken* place ; those rudiments, 
the second pre-oral and first post-oral folds, instead of being nearly parallel, are placed 
at right angles to each other, the dorsal end of the maxillary fold abutting against the 
front of the dorsal end of the mandibular fold. 
The distal ends of these folds are much enlarged ; this is best seen in a lower view 
(fig. 4). They both project inwards, but the foremost pair are far apart; the man- 
dibular folds have an extensive symphysis, and form a very solid chin. 
The pre-oral cleft ( l.cl .) is shallow and imperfect ; the first post-oral cleft is a large, 
long, sinuous, and perfect slit ; it passes downwards and backwards, a definite part of 
the root of the next fold lying between it and the ear-capsule. 
The next or hyoid fold is now, relatively, much smaller, and like the other three — 
for there are three branchial folds now — it is curved forwards ; the three other clefts 
are much smaller than the first post-oral ; they are arcuate, and bend forwards. 
The small hyoid fold does acquire some cartilage above, but the other three folds 
remain membranous, and they are now at their fullest development. 
In the first stage (fig. 1 ) the ear-sac was embraced by the foot of the hyoid fold, and 
