DEVELOPMENT OF THE SKULL IN THE CROCODILIA. 267 
These parts—the fronto-nasal and maxillo-palatine folds (f.2.p, mx.p)—are in front 
of the hypoblastic layer of the embryo, and therefore have to be considered, in the 
present state of our knowledge, merely as additions or outgrowths to the true visceral 
folds, which are lined with the ower embryonic layer. 
In the preparation figured, the myelon (my) has been cut through, and also the 
notochord (nc) and investing mass (iv); also the mandibular folds and hyoid arches 
(mn, hy); these arches are now becoming cartilaginous. The notochord can be seen 
shining through the hinder part of the floor of the head; in front of it there is an 
opening partly filled by a sort of glandular structure—the rudimentary pituitary body 
(fig. 2, py). From the middle of the mandibular fold, right and left, a membranous fold 
appears with a free sinuously emarginate hinder edge, this fold, which runs up to the 
fronto-nasal fold, is the epiblastic lining of the palate; the higher stratum of mucous 
membrane behind it is composed of hypoblast, which reaches in front to the notch in 
which the pituitary body lies, and on each side passes into the first cleft (cl). In 
front of the first cleft the hypoblast unites with epiblast, ending there. The second 
cleft (cl?) is seen behind the hyoid fold (hy); the other folds and clefts were not figured : 
these parts had been cut away. 
Inside, the solid maxillo-palatine folds (ma-.p), right and left, are thick and bulbous; 
these swellings are the rudiments of the lower or hard palate, which is so greatly 
developed afterwards. When the mucous membrane and palatal skin had been 
removed (Pl. LXIII. fig. 2), then the rudimentary chondrocranium was displayed; the 
mandibular arches only (mn) were left in section in this preparation. The notochord 
(nc) seems to be a straight, horizontally placed rod, but its position is really oblique 
(see 2nd stage, Pl. LXIII. fig. 7, ne). It lessens gradually to its fore end, where it is 
somewhat bulbous, and shows but little tendency to become moniliform. On each side 
of the notochord the parachordal cartilage (investing mass, 7v) is becoming solid; it 
is almost regular in width, but is indented by the corresponding auditory capsule (aw), 
itself now just becoming hyaline. 
The front margin of the basal plate is emarginate, bounding, as it does, the pituitary 
body behind; from each moiety a small horn of less consistent tissue is seen to grow 
forwards with an outward curve; this bovicorn structure is the prochordal part of the 
basis cranii, in rudiment; the horns are the “ trabecule cranii.” Here the structure is 
quite like what is seen in the Axolotl (Phil. Trans. 1877, pl. 22. fig. 1), except that in 
that type the “horns” solidify first; this is also the case in the larval Lamprey. At 
this stage the trabecule are very slight additions to the proper axial skeleton; they just 
help to support the first vesicle of the brain. 
2nd Stage. Embryos of Alligator mississipensis, 13 inch long (head 4 lines, 
body 6, tail 8). 
At this stage the embryo has not only taken on the pentadactyle character, it is also 
