266 PEOP. W. K. PAEKBK ON THE STRUCTUEE AND 



Thus in the axial and neural regions we have a ^amchordal 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 sujier- (or ^jre-) 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. 



1st Stage. Embryos q/" 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, moi'e than fifty somatomes were developed, and the limbs were in the 

 condition of trowel-shaped paddles (PI. LXII. fig. 1). The ventral laminte were 

 imperfect, so that the heart (A) protruded below the throat, and the umbilical vessels 

 (?«. 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 {ol, c, au) were already formed, and the 

 involution of the latter [aq.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 (cZ'"'), the first of which was short, cresccutic, 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 (PI. LXll. fig. 2), the mouth (hi) is bordered behind by the solid 

 mandibular folds (mn) ; behind these are the hyoid folds (hy), 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 (ir'"). 

 The open oral space (m) has, right and left of it, a solid semioval fold, the maxillo- 

 palatine (mx.])), 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 (ol) ; 

 this double middle fold is the " fronto-nasal process " (f.n.p). 



When the inferior (postoral) arches are removed, and the head viewed from below 

 (PI. LXllI. fig. 1) the form of the folds that finish the face in front is better seen. 



