The first of these divisions forms a space of a considerable size, with epithelial walls of somewhat short columnar cells ([fig. 381], 1pp). It is situated close to the eye, and presents a rounded or sometimes a triangular figure in section. The two halves of the cavity are prolonged ventralwards, and meet below the base of the fore-brain. The connection between them appears to last for a considerable time. These two cavities are the only parts of the body-cavity within the head which unite ventrally. The section of the head-cavity just described is so similar to the remaining sections that it must be considered as serially homologous with them.
The next division of the head-cavity, which from its position 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 epithelium.
Fig. 382. Horizontal section through the penultimate visceral arch of an embryo of Pristiurus.
ep. epiblast; vc. pouch of hypoblast which will form the walls of a visceral cleft; pp. segment of body-cavity in visceral arch; aa. aortic arch.
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 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 constrictor superficialis and musculus interbranchialis[246]; 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. [509]).
Marshall finds that the ventral portion of this cavity, where its two halves meet, becomes separated from the remainder. The eventual fate of this part has not however been followed. Each dorsal section acquires a cup-like form, investing the posterior and inner surface of the eye. The cells of its outer wall subsequently give rise to three sets of muscles. The middle of these, partly also derived from the inner walls of the cup, becomes the rectus internus of the eye, the dorsal set forms the rectus superior, and the ventral the rectus inferior. The obliquus inferior appears also to be in part developed from the walls of this cavity.