140 ON THE STRUCTURE OF THE SKULL. 



being only secondary. The auditory nerve is developed in the 

 blastoderm adjacent to the third cerebral vesicle, so that the 

 three pairs of sense-capsules do not correspond with the three 

 primary cerebral vesicles. 



While these changes have been going on in the proper 

 cranial portion of the embryo, the rudiments of the face have 

 made their appearance under a very singular guise. As the 

 homologues of the dorsal laminae in the head have grown up- 

 wards to inclose the cephalic cavity, so, plates, which correspond 

 with the visceral laminae of the trunk, have grown downwards, 

 to constitute the posterior walls of the buccal, pharyngeal, and 

 cervical regions. These visceral plates, however, do not remain 

 entire and undivided, as do those of the trunk, but grooves 

 appear in them, directed transversely to the axis of the trunk, 

 and, the grooves deepening, eventually become converted into 

 slits — the visceral clefts — which open into the pharyngeal cavity, 

 and bound corresponding visceral arches. The first slit is 

 situated immediately below and in front of the auditory sac, 

 and separates the first and second visceral arches, — the anterior 

 boundary of the former being determined by the edges of a 

 depression of the integument which wall eventually become the 

 buccal cavity (Fig. 57, C). A third, fourth, and fifth visceral 

 arch are developed in successive order behind the first and 

 second (Fig. 57, D, and Fig. 32, F) ; but as they are of no great 

 moment in reference to the human skull, our attention may be 

 confined to the latter. 



It is particularly worthy of notice that, from the moment at 

 which it is discernible as a distinct part, the root of the first 

 visceral arch passes into the rudimentary cranium below, and in 

 front of, the forepart of the auditory sac, while the root of the 

 second is attached below and behind that sac. We shall find 

 that the parts developed within these arches retain the same posi- 

 tion in the adult state ; so that any hypothesis which involves 

 the supposition of an extensive change of place of these parts in 

 the course of development is, ipso facto, unworthy of consideration. 



Both the first and second visceral arches are connected with 

 that part of the cranium which lies behind the flexure ; but the 

 inflected portion of the cranium in front of the bend exhibits, 



