OF THE HKAD IX THE BATRACHIA URODKLA. 



109 



tocbondrifv behind ; tlie occipital ai'cli is fused on above the vagu.s. 

 The floor of the sknll widens out considerably from the occipital 

 segment forwards to allow space for the bixiin, and the preoccipital 

 arch is placed opposite the point where the occipital parachordal 

 plate meets the anterior pai-achordal plate, a point not yet chon- 

 drified. Although it appears to chondrify in continuity with the 

 auditory capsule, the prajoccipital arch is probably serially homo- 

 logous with the neural ai-ches, as already suggested by Miss Piatt. 



nch 



Reconstruction, seen from above, of the licaJ and anterior trunk-region. Tlie dorsal 

 ends of tlie gill-arclies are shown, also the ventral region of the myotomes. 

 Cranial nerve-roots are indicated on the right side. 



Its first appearance as a procartilaginous rudiment is seen in text- 

 tig. 37, taken from a still younger larva, in the septum between the 

 2nd and 1st myotomes. At this stage the ventral portion of the 

 first myotome (2nd somite) is still present. The auditory capsule 

 itself is scarcely recognisable and quite without cartilage. 



In the next younger stage, text-figs. ,38 and 39. the pra^occipital 



