CHONDROCRANIUM OP THE LARVAL HERRING. 



1223 



slight restriction where it meets Meckel's cartilage. At the 

 lower part of this restriction the two cartilages are divided to 

 indicate a coming division between them. 



Meckel's cartilage is very much as in the smaller 10 mm. stage, 

 wide at the base and tapering to a thin, horizontally flattened 

 cartilage at its anterior end. 



The two foramina of the VHth, facial, nerve {i. e. in the 

 hyomandibular and in the ethmoid plate) are unchanged, 



cl. The 30 mm. Stage. 



The following description is based on the study of transverse 

 sections of a 30 mm. specimen of Clupea hat^engus and on a 

 reconstruction of it (text-fig. 7). 



The median ridge in this stage runs to the extreme anterior 

 end of the ethmoid plate, the top bulging latei-ally considerably ; 

 near the posterior end of this ridge it divides to enclose the 

 anterior end of the brain. Just below the top bulge there is the 

 foramen of the 1st nerve. 



Text-figure 7. 



Lateral view of chondrocranium. 30 mm. Stage. X 20. 



Th e ethmoid plate runs back from this at first as in the 



previ ous stage, being nearly fl.at. It then tapers to a long 



median point, on each side of the base of which are slight knobs, 



the remains of tlie Trabeculae Cranii, which are now absent. A 



ventral view of this is shown in text-fig. 8. 



From the cartilage enclosing the anterior end of the brain the 

 paired post-orbital processes extend back as before. The bridge 

 over the brain and between these (the epiphysial bar) is 

 unchanged. From the posterior end of this bar there runs a 

 process, along the top of the brain, to meet the cover to the 

 posterior end of the brain. 



The auditory capsules are now completely enclosed by cartilage. 

 The processes which in the 20 mm. stage grew back from close 

 behind the post-orbital processes are now broken, to give long 

 sloping slits in the upturned sides of the ethmoid plate. Part of 

 the posterior end of the hind brain and the anterior end of the 

 nerve-chord are entirely roofed over by cartilage. 



81* 



