506 



Mil. N. A. mackintosh: on the CHONDHOCIIANIUM 



The notochord still reaches well forward and maintains a 

 marked flexure (text-fig. 4). Another peculiarity to be mentioned 

 here is that there is no sign of a fenestra hypophyseos which is 

 normally present in a tropibasic chondrocranium at this stage. 

 The trabeculto nre still closely approximated along the whole of 

 their length. The parachordals are now well developed and form 

 broad, saucer-shaped bases to the auditory vesicles. 



The visceral skeleton has maintained its advance over the 

 cranial elements. The lower jaw has now attained a considerable 

 length, but there is sfcill no vestige of an upper jaw. In this 

 peculiarity the development resembles that of the larval Clupea 



Text-figure 4. 



Stage 3. Lateral view of chondrocranium. 



(Wells, 1922) in which the upper jaw does not appear until the 13 

 mm. stage. The temporal cartilage (text-fig. 4) has lengthened, 

 but has otherwise undergone no change except for the fact that 

 the hyomandibular portion is better defined. There is at present 

 no foramen for the hyomandibular branch of the Vllth nerve. 

 It appears that the nerve becomes enclosed in the cartilage later 

 on. The cartilage joining the coratohyal to the temporal is now 

 constricted to a narrow stylohyal. Considerable development lias 

 taken place in the branchial bars (text-fig. 6). There is a median 

 copula communis to which the ceratohyal and the first three 

 branchial bars are attached. The fourth bar is free and the fifth 



