THE MUSCULAR SYSTEM. 559 



is so similar to the remaining sections that it must be cousidered 

 as serially homologous with them. 



The next division of the head-cavity, which from its position mav 

 be called the mandibular cavity, presents 

 a spatulate shape, being dilated dorsally, 

 and produced veutrally into a long thin 

 process parallel to tb.3 hyomandibular gill- 

 cleft (fig. 20, pp}. Like the previous space 

 it is lined by a short columnar epithelium. aa __. 



The mandibular aortic arch is situated 

 close to its inner side (fig. 381, -pp). After 

 becoming separated from the lower part FIG. 382. HORIZONTAL SEC- 

 (Marshall), the upper part of the cavity TION EOUGH THE *ENULTI- 



.'' I 1 . 1 , J MATE VISCERAL AKCH OF AN EM- 



atrophies about the time ot the appear- BRY o or PRISTIURUS. 



ance of the external gills. Its lower part ep. epiblast; vc. pouch of 



also becomes much narrowed, but its walls typoblast which will form the 



c -, 11 nM walls of a visceral cleft; />. 



of columnar cells persist. Ihe outer or segment of body . cavity in 4/ 8 . 



Somatic wall becomes very thin indeed, ceralarch; aa. aortic arch, 

 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 fundamentally 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 posterior parts of the head become much reduced like 

 those in its anterior part, though at rather a later period. 



It has been shewn that the divisions of the body cavity in the 

 head, with the exception of the anterior, early become atrophied, 

 not so however their walls. The cells forming the walls both of 

 the dorsal and ventral 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. 



Its further history is very interesting. In my original account of 

 this cavity (No. 292, p. 208) I stated my belief that its walls gave 

 rise to the eye-muscles, and the history of this process has been to 

 some extent worked out by Marshall in his important memoir (No. 



Marshall finds that the ventral portion of this cavity, where its 

 two halves meet, becomes separated from the remainder. The eventual 



i Vide Vetter, "Die Kiemen mid Kiefermusculatur d. Fische." Jcnaische Zcit- 

 sclirift, Vol. vn. 



