DEVELOPMENT OF THE SKULL IN THE CROCODILL\. 267 



These parts — the fronto-nasal and maxillo-palatine folds {f.n.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 Icnowledge, merely as additions or outgrowths to the true visceral 

 folds, -which are lined with the lower embryonic layer. 



In the preparation figured, the myelon {my) has been cut through, and also the 

 notochord {nc) and investing mass iiv) ; 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 ((?/'). In 

 front of the first cleft the hypoblast unites with epiblast, ending there. The second 

 cleft {cF) 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 {mx.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 (PI. 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, PI. LXIII. fig. 7, nc). 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, iv) is becoming solid ; it 

 is almost regular in width, but is indented by the corresponding auditory capsule {au), 

 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 " trabeculse cranii." Here the structure is 

 quite like what is seen in the Axolotl (Phil. Trans. 1877, pi. 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 trabeculse 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, 1\ 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 



