Section 37. but As Certainly As We Have No Such Metameric

segmentation, as this older view implies, in the brain-case of the frog, so quite as certainly is metameric segmentation evident in its branchial arches. We have the four gill slits of the tadpole and their bars repeating one another; the hyoid bar in front of these is evidently of a similar nature; and that the ear drum is derived from an imperforate gill slit is enforced by the presence of an open slit (the spiracle) in the rays and dog-fish in an entirely equivalent position. Does the mouth answer to a further pair of gill slits, and is the jaw arch (palato-pterygoid + Meckel's cartilage) equivalent to the arches that come behind it? This question has been asked, and answered in the affirmative, by many morphologists, but not by any means by all. The cranial nerves have a curious similarity of arrangement with regard to the gill slits and the mouth; the fifth nerve forks over the mouth, the seventh forks over the ear drum, the ninth, in the tadpole and fish, forks over the first branchial slit, and the tenth is, as it were, a leash of nerves, each forking over one of the remaining gill slits. But this matter will be more intelligible when the student has worked over a fish type, and need not detain us any further now.