CHONDROCEANIUM OF THE LARVAL HERRING. 



1225 



end a pair of small backward-growing protrusions have appeared. 

 The median process running along the top of the brain is very 

 much thinner, especially near the posterior end. 



The long sloping slits in the upturned sides of the posterior 

 part of the ethmoid plate — mentioned as present in the 30 mm. 

 stage — are now closed at the upper and anterior end, leaving a 

 large fox^amen. The roof over the posterior end of the brain is 

 now complete. 



The anterior end of the palato-pterygoid is practically 

 unchanged, the posterior end being somewhat enlarged to form 

 an upturned knob. There is a slight restriction where it meets 

 the hyomandibular mass. 



Text-figure 9. 



Lateral view of chondrocraniuni. 40 mm. Stage. X 20. 



The articulation of the hyomandibular with the cranium is no 

 longer complete, being broken in the middle. The hyomandibular 

 mass has changed very considerably — changes which are even more 

 marked in the .50 mm. stage, — and it is perhaps best to describe 

 them in the next section. 



Considerable ossification is commencinsr. 



f. The 50 iwrn. Stage. 



[See text-figure 10.] 



The nasal septum appears at this stage at the base of the 

 rostrum ; apart from this the anterior part of the ethmoid plate 

 remains unchanged. The post- orbital processes are again more 

 massive, and the process running medianly from the epiphysial 

 bar to the posterior end of the cranium is, as it originally was, 

 incomplete. 



The retro-articular process on Meckel's cartilage is now con- 

 spicuously developed. 



No other diff"erences have appeared except in those cartilages 

 concerned in the suspension of the jaws. 



Text-fig. 11 shows the suspensory apparatus of Clupea 

 harengus and of Amia calva (from Torsten Pehrson, 1922) for 



