NOTES ON ELASMOBRANCH DEVELOPMENT. 571 
tinuous with each other. The mouth soon widens and shortens 
(figs. 8, 10, 12) 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. 
