314 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



with one another first towards their morphologically anterior ends 

 a vacuity persisting for a time beneath the infundibulum. As might 

 be inferred from the study of Fig. 54 (p. 93), which shows how the 

 floor of the thalamencephalon gradually assumes its definitive 

 horizontal position, the trabecular portion of the cranial floor pari 

 passu swings forwards and comes to be more nearly in line with the 

 parachordal portion. The displacement of the trabeculae which we 

 have associated with the exaggerated cerebral flexure is then a 

 temporary phenomenon which tends to become corrected during 

 subsequent development. The cartilage formed by the fusion of the 

 anterior ends of the trabeculae becomes prolonged forwards between 



' ' 







B 



FIG. 157. Diagrams illustrating the early development of the chondrocranium of Birds. 

 (Based on figures by Sonies, 1907.) 



A, Chick, 11 mm. ; B, Duck, 13 mm. ; C, Duck, 15 rnm. ; D, Duck, 14 mm. ; E, Chirk, lii mm. 

 o.c, auditory capsule ; nor, arrorhordal cartilage ; i.o.s, interorbital septum ; wo, m-sotic rartila^f ; ".", 

 neural arch ; p, polar cartilax-- ; //./, pituitary foramen ; p.b.f, posterior kisu-rnuial fontanelle ; p.c, 

 j.aiaclioidal ; tr, trabecula. 



tin- olfactory organs as a rod of cartilage, the rostral cartilage, which 

 represents the ventral edge of the internasal septum. 



The outer wall of the olfactory capsule appears as an, at first 

 independent, piece of cartilage on the anterolateral side of tin- 

 olfactory organ \vliidi inaxlually spreads round the organ in question 

 and becomes <;ont in nous with the rest of the cranium. 



It is unnecessary to follow out. in detail the modelling of the 

 definitive cranium but, it should be noticed that the cranial cavity 

 gradually I '<<<, nuis roofed in by the upgrowth of its side walls, and 

 that, in some cases at least (Pristiurus), this lootin^-in process 

 becomes romj,i<.t,Mi tirst in UK- region between th i uu<lit<>i \- raj.snlt's. 



of interest \\IHMI oorrelated \\ith the persistem f this 



portion of the chondrocranial roof in /'rotopterus suggest in- that 



