314 EMBRYOLOGY OF THE LOWEE VERTEBRATES oh. 



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 



wnS 



act: ^ 



\^pc. 



-It- 

 ,P 



. r 



•3't: 

 B 



pc 



<*>* 



acn 



Pf 



-pc 



Vj^^pc. 





\? c ^*i^ c - 



V 



Fig. 157. — Diagrams illustrating the early development of the chondroeranium of Birds. 

 (Based on figures by Sonies, 1907.) 



A, Chick, 11 mm. ; B, Duck, 13 mm. ; C, Duck, 15 mm. ; D, Duck, 14 mm. ; E, Chick, 12 mm. 

 a.c, auditory capsule ; acr, acrochordal cartilage ; i.o.s, interorbital septum ; mo, mesotic cartilage ; n.a, 

 neural arch ; p, polar cartilage ; p.f, pituitary foramen ; p.b.f, posterior basicranial fontanelle ; p.c, 

 parachordal ; tr, trabecula. 



the 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 the 

 olfactory organ which gradually spreads round the organ in question 

 and becomes continuous 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 becomes roofed in by the upgrowth of its side walls, and 

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

 becomes completed first in the region between the auditory capsules. 

 This fact is of interest when correlated with the persistence of this 

 portion of the chondrocranial roof in Protopterus — suggesting that 



