266 PROF. W. K. PARKER ON THE STRUCTURE AND 
Thus in the axial and neural regions we have a parachordal tract with a pro- 
chordal tract in front of it; whilst, below, we have the postoral (visceral) arches, the 
foremost of which, the mandibular, is finished in front by swper- (or pre-) oral out- 
growths. Moreover, the visceral arches must not be confounded with the costal arches ; 
these latter are found in the outer layer of the body-wall, the “‘ somatopleure,” whilst 
the visceral arches are developed in the inner layer, or “splanchnopleure.” The fact 
that these two layers are only separate for a short time in the pharyngeal region, does 
not alter their real nature; the difference between the two sets of arches is funda- 
mental, and only in the lower types (Ichthyopsida) do arches arise in the superficial 
layer of the pharynx. 
Ist Stage. Embryos of Alligator mississipensis, Daudin, 11 lines long 
(head 3, body 5, tail 3). 
My smallest Crocodilian embryo, measured along its coils, was less than an inch in 
length, and served well for comparison with the early embryos of the other Reptilian 
forms already figured and described. The mesocephalic flexure was perfect, the visceral 
clefts visible, more than fifty somatomes were developed, and the limbs were in the 
condition of trowel-shaped paddles (Pl. LXII. fig. 1). _ The ventral lamine were 
imperfect, so that the heart () protruded below the throat, and the umbilical vessels 
(w.v) came from an open abdomen. ‘The divisions of the brain were clearly seen from 
the outside; the hind brain (C’) was large and long, the mid brain (C’) large and 
protuberant and ending the axial line, and the fore brain had already given off the 
rudiments of the hemispheres (C', C”). 
The nasal sacs, eyeballs, and auditory capsules (0/, e, aw) were already formed, and the 
involution of the latter (aqg.v) was visible. There were four pairs of visceral folds behind 
the mouth, the mandibular, hyoid, and first and second branchial (mn, hy, br»”); these 
were separated by three clefts (¢/'”), the first of which was short, crescentic, and obscure, 
and the other two large and open. Already the first and second visceral folds had gained 
largely on the other two, being thick and solid, whilst the two branchial folds were very 
slender and small, ready to lose their distinctness in the folds of the neck. 
Seen from below (Pl. LXII. fig. 2), the mouth (m) is bordered behind by the solid 
mandibular folds (mm); behind these are the hyoid folds (Ay), which have an opercular 
fold, as in fishes. There is a wide open space between them and the first branchial fold, 
and then a smaller cleft, right and left, between the first and second branchials (d7"'’). 
The open oral space (m) has, right and left of it, a solid semioval fold, the maxillo- 
palatine (ma.p), then there is a clear chink on each side, and, in front, a fan-shaped 
lobe, cleft in the middle, and, flanking each lobe, the right and left olfactory sacs (of) ; 
this double middle fold is the “ fronto-nasal process ” (f.7.p). 
When the inferior (postoral) arches are removed, and the head viewed from below 
(Pl. LXIII. fig. 1) the form of the folds that finish the face in front is better seen. 
