246 ADAM SEDGWICK. 



tinuous with each other. The mouth soon widens and shortens 

 (figs. 8, 10, 13) until it attains its adult form. 



The mandibular arch is at first directed almost from before 

 backwards (figs. 5, 6, 7), and its anterior end is under the 

 mid-brain. 



The hyoid arch is also directed very much backwards, 

 though not so much as the mandibular ; and its anterior 

 (dorsal) end is well in front of the auditory sac (fig. 7). 



The branchial arches are also directed backwards, but the 

 inclination is less in the posterior arches than in the anterior 

 (fig. 9). 



The question now arises, what is the meaning of this back- 

 ward direction of the visceral arches ? The only answer that 

 I can suggest to this question is that the same cause which 

 has produced the flexure of the brain, and of the front end of 

 the notochord, has affected the arches. If this is so the cranial 

 flexure should really be called cephalic flexure, for it affects 

 not merely the brain, but all the organs of the head. 



To account for this flexure we must either suppose that 

 there has been a great forward extension of the dorsal anterior 

 end of the head, which would carry the dorsal ends of the 

 arches forward, and, if the anterior end of the notochord and 

 the infundibulum, i. e. the anterior end of the cranial axis, 

 remained fixed at the front end of the mouth, would also 

 cause the flexure of the brain and anterior part of the noto- 

 chord ; or that there has been a great shrinking of the ventral 

 parts of the head just behind the mouth. If either of these 

 views is correct, it necessarily follows that the mouth was 

 originally a nearly vertically directed slit looking straight 

 forward. It may even have extended on to the dorsal surface. 



The early slit-like form of the mouth is very remarkable, 

 and may be regarded as being in favour of the view that the 

 mouth is derived from the anterior part of the slit-like blasto- 

 pore, though I admit that this does dot constitute a very 

 powerful argument. 



The extension forward of the first rudiment of the mouth 

 into the pituitary pocket is also very remarkable. 



