1218 ME. F. B, WELLS ON THE MORPHOLOGY OF THE 



At the posterior end of this foramen the processes widen 

 laterally and turn up sharply. The median part forms a tube 

 enclosing the notochord. 



From near the top of the lateral upturned process there 

 grows down a cartilage, laterally thin, Avhich soon elongates to 

 form a large flat plate, slating out at the top. This is the hyo- 

 mandibular. Close below its fusion with the cranium there is 

 a foramen, of the hyoid branch of the Vllth (facial) nerve. 



At the bottom of this plate and slightly in fi'ont of its fusion 

 with the cranium there is a narrow elongated slit. From the 

 posterior end of this hyomandibular there grows downwards and 

 forwards a thin cartilage. This runs along the lower side of the 

 hyomandibular, and then forwards and upwai'ds. The halves (it 

 is, of course, like the hyomandibular, paired) meet some way in 

 front of the anterior end of the ethmoid plate ; they are not 

 fused, but articulate. 



(The naming of the cartilages is based on study of later 

 stages.) 



There is at this stage no trace of an upper jaw (i. e. the 

 palatal, pterygoid, and quadrate cartilages), the only roof to the 

 mouth being the ethmoid plate. 



A specimen of 6*5 mm. was examined, and showed only a few 

 slight differences ; the large upturned processes are distinctly 

 smaller, and the narrow elongated slit in the hyomandibular mass 

 is closing up. The fusion of this mass with Meckel's cartilage is 

 still placed as in the younger stage. 



In a 7*5 mm. specimen it was found that the large tipturned 

 processes had diminished considerably in size, being very little 

 larger, comparatively, than in the 10 mm. stage. The narrow 

 elongated slit in the hyomandibular has entirely closed up, and 

 the fusion of the posterior process from Meckel's cartilage to the 

 hyomandibular has moved forwards, being now situated directly 

 below tlie anterior end of the fusion of the latter to the cranium. 



Text-fig. 2C shows a reconstruction of a 7'8 mm. specimen ; the 

 upturned processes have disappeared entirely, and the fusion of 

 Meckel with the hyomandibular has moved right forwards to the 

 anterior end of the latter, and its fusion with the cranium is as 

 before. 



b. The 10 mvi. Stage. 



This description of the chondrocranium is based on the study 

 of transverse sections in all nine series, on whole mounts of 

 specimens of Clupea harengus varying in length from 9 to 

 11 mm., and on reconstructions made from a 10 mm. specimen 

 (text-figs. 3 & 4). 



In front the ethmoid plate is still thin, but it has flattened 

 out considerably, being at first dorsally convex and, further 

 back, concave. In front this ethmoid plate is very broad, 

 measuring nearly -5 mm. across ; it runs back for nearly one- 

 third of the length of the head before dividing to sive off the 



