678 THE HEAD-CAVITIES. 



may be called the mandibular cavity, presents a spatulate shape, 

 being dilated dorsally, and produced ventrally into a long thin 

 process parallel to the hyomandibular gill-cleft (fig. 20, pp}. 

 Like the previous space it is lined by a short columnar epi- 

 thelium. 



The mandibular aortic arch is situated close to its inner side 

 (fig. 381, 2pp). After becoming separated from the lower part 

 (Marshall), the upper part of the cavity atrophies about the time 

 of the appearance of the external gills. Its lower part also 

 becomes much narrowed, but its walls of columnar cells persist. 

 The outer or somatic wall becomes very thin indeed, the 

 splanchnic wall, on the other hand, thickens and forms a layer 

 of several rows of elongated cells. In each of the remaining 

 arches there is a segment of the original body-cavity fundamen- 

 tally similar to that in the mandibular arch (fig. 382). A dorsal 

 dilated portion appears, however, to be present in the third or 

 hyoid section alone (fig. 20), and even 

 there disappears very soon, after being 

 segmented off from the lower part 

 (Marshall). The cavities in the pos- 

 terior parts of the head become much 

 reduced like those in its anterior part, 

 though at rather a later period. FlG . 382 . HORIZONTAL 



It has been shewn that the divi- SECTION THROUGH THE PEN- 

 ULTIMATE VISCERAL ARCH OF 



sions of the body-cavity in the head, AN EMBRYO OF PRISTIURUS. 

 with the exception of the anterior, e p. epiblast; vc. pouch of 

 early become atrophied, not so how- hypoblast which will form the 



walls of a visceral cleit ; //. 

 CVer their walls. The cells forming segment of body-cavity in vis- 



the walls both of the dorsal and ven- ceral arch ; aa ' aortic arch ' 

 tral sections of these cavities become elongated, and finally 

 become converted into muscles. Their exact history has not 

 been followed in its details, but they almost unquestionably 

 become the musculus contrictor superficialis and musculus inter- 

 branchialis 1 ; and probably also musculus levator mandibuli and 

 other muscles of the front part of the head. 



The anterior cavity close to the eye remains unaltered much 

 longer than the remaining cavities. 



1 Vide Vetter, " Die Kiemen und Kiefermusculatur d. Fische." Jenaische Zclt- 

 schrift, Vol. vn. 



