1222 



ME. F. R. WELLS ON THE MORPHOLOGY OF THE 



protrusions and come down close to the median ridge. About 

 half-way along these post-orbital processes a bridge is formed 

 between them, which gives off two median thin processes, one 

 forwards and one backwards. This bridge roofs over part of the 

 anterior end of the brain (epiphysial bar). 



Arising from the anterior end of the upturned sides of the 

 ethmoid plate, from which the post-orbital processes arise, are 

 paired backward growing pieces of cartilage ; these meet processes 

 growing up from the sides of the posterior end of the ethmoid 

 plate, and enclose the anterior end of the auditory capsules. 

 From the top of these a bridge grows to cover part of the 

 posterior end of the brain. The posterior end of the auditory 

 capsules is enclosed as in the 10 mm. stage. Tlie cartilage 

 enclosing the notochord is as before. 



Articulated with the sides of the anterior end of the ethmoid 

 plate are small paired cartilages — the early beginnings of the upper 

 jaws (the palatal part of it). This appears first at about 13 or 



Text-figure 6. 



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



14 mm . They run back as far as the anterior end of the pituitary 

 foramen. They have no connection whatever with either the 

 hyomandibular or with Meckel's cartilage. This is very similar 

 to the 10 mm. Amiurus, in which the palatal arises separately 

 close to the ethmoid plate and the pterygo-quadrate grows 

 forwards from the hyomandibular to meet it. At this stage, 

 however, there is no trace of any forward growth from the 

 hyomandibular. The palatal arises separately similarly m 

 tSyngnathus (8 mm.). . 



The hyomandibular is articulated with the cranium, and m no 

 place is it fused with it; it is a considerably larger mass of 

 cartilage, comparatively, than in the 10 mm. stage. The part 

 articulating Avith the ceratohyal has grown back and away from 

 the main mass, forming even more conspicuously a stylohyoid 

 cartilage. Anterior to the base of this process a long pointed 

 piece of cartilage grows forwards. 



The hyomandibular is considerably elongated, and there is a 



