CHONDROCRANIUM OF THE LARVAL HERRING. 



1219 



Trabecul^B Oranii. The pituitary foramen is now about -45 mm. 

 long and half as wide. Just anterior to the posterior end of this 

 foramen these processes (the Trabeculfe Cranii) widen laterally 

 as before, and turn up sharply to send forwards the long, slender 

 post-orbital processes. (These might also be called the supra- 

 orbital processes, but the former name is chosen as it emphasises 

 the fact that they grow forwards from behind, and that they lie 

 between the eyes and the brain and not over the eyes.) Small 

 protrusions, corresponding to the large upturned processes of the 

 5 mm. stage, turn back from the ethmoid plate as if to meet 

 these post-orbital processes. The orbit is thus, at this stage, 

 incomplete, as it is in the 10 mm. Syngnathus, whereas in the 

 8 mm. Amiurus and the second week Salmo it is complete. 



In the flat plate betw-een the pituitary foramen and the post- 

 orbital processes are the foramina of the Vllth nerve. 



The anterior end of the notochord lies directly behind the 

 pituitary foramen, and it is enclosed in cartilage for the first 



Text-figure 3. 



Lateral view of ctondrocrauium. 10 mm. Stage. X 55. 



'5 mm. of its length. On each side of this are the auditory 

 capsules, enclosed at their posterior end b}^ cai-tilage. The brain 

 and nerve-cord are in no place completely roofed over, the 

 nearest approach to this being two thin, high processes lying 

 laterally to and nearly level with the top of the nerve-cord ; 

 these arise from the cartilage enclosing the notochord, near the 

 posterior of the head. In them is the foramen of the Xth nerve 

 (vagus). The 10 mm. Syngnathus, 8 mm. Amiurus, and second 

 week Salmo, all have the posterior end, at least, of the brain 

 roofed over. 



At this stage there is still no trace of an upper jaw. Meckel's 

 cartilage protrudes conspicuously beyond the ethmoid plate ; the 

 two halves are narrow but deep. The jaw widens rapidly until 

 it is nearly as wide as the ethmoid plate, and then narrows slowly 

 until it meets the hyomandibular. The two ai-e still definitely 

 fused, Meckel's cartilage extending as a long forward growth 

 from the anterior end of the hyomandibular. 



