de^t;lopmext of the skull in the CEOCODILIA. 273 



Part of a section, much like this, is figured in PI. LXVIII. fig. 2 ; here the columella 

 (co) dents the auditory capsule (cm), the epihyal is just missed, and the head and 

 shoulders of the ceratohyal (c.hi/) are cut obliquely across. 



But another section from a higher plane (PI. LXVIII. tig. 1) is the most instructive ; 

 here the columella or pharyngo-hyal fits into a hole, the fenestra ovalis, which opens 

 into the vestibule (vb) ; on the inside that cavity is growing into its cochlear diverti- 

 culum (clil). Here the small curved epihyal (e.Iiy) is cut across, and also (outside 

 and below it) the head and shoulders (obliquely) of the ceratohyal [c.hy). The 

 fore part of the investing mass (iv), the body of the quadrate (g), and the first cleft 

 (c/') are seen, as cut across, in this figure. 



In a lower section (PI. LXVIII. fig. 4) the quadrate cartilage is seen sending its long 

 orbital process inwards and forwards ; behind it the ceratohyal {c.hy) is cut across. In 

 another figure (fig. 6) the ceratohyal {c.hy, ar) is seen as cut across at its junction with 

 the angular part of the mandible. 



In a still lower section (PI. LXVIII. fig. 7) Meckel's cartilage {mk) is cut across, and 

 behind it, at the middle of the floor of the face, we see the basihyo-branchial plate 

 (b.h.br) with the rudimentary first ceratobranchials {c.br^) ; these are gently curved, 

 rounded rods of cartilage. In the lowest of these sections (PI. LXVIII. fig. 8) the 

 larynx {Ix) is cut along ; and here the curved thyrohyal or first ceratobranchial is cut 

 in its curve so as to look like two pieces. 



The very ichthyic condition of these arches seen in this stage will be found to be 

 greatly transformed in the stages that follow. 



The next stage will illustrate the further growth of the chondrocranium and visceral 

 arches, and the first definite appearance of the investing bones. 



ord Stage. Embryos q/' Crocodilus palustris, yrom If inch to 2\ inches. 



a. Chondrocranium. 



This stage (PI. LXII. figs. 5-9, PI. LXIV. figs. 5-11, and PI. LXVIII. figs. 10, 11) 

 follows very closely upon the last, but belongs to another species and genus. 



I shall give all the details of the chondrocranium in the next stage, but in this 

 the main things will be noticed ; the cartilaginous framework is already perfectly 

 formed. 



The notochord (PI. LXIV. figs. 5-8, nc) is now closely embraced by the parachordal 

 tracts, behind, and the cartilage has formed a semilune, below, half embracing it; this 

 is the transversely oval occipital condyle {oc.c). Then for three fifths of its extent 

 the notochord is naked below, but it gains a superior position in front, the basal 

 (parachordal) tracts having coalesced again beneath it. It escapes once more at the end, 

 where it projects, inwards and upwards, into the neat circular pituitary space {py). The 

 basal cartilage is dilated behind and in front of the auditory capsule, but is greatly 



2u2 



