DEVELOPMENT OF ELASMOBRANCH FISHES. 207 



times triangular figure in sections (PL xiv. fig. 7, 9 6 and IG 6, 

 I. 2^P-)' The ophthalmic branch of the fifth nerve passes close 

 to its superior and outer wall. 



Between stages I and K the anterior cavities of the two 

 sides are prolonged ventralwards and meet below the base of 

 the fore-brain (PI. xiv. fig. 8, i.pj).). The connection between 

 the two cavities appears to last for a considerable time, and still 

 persists at the close of stage L. The anterior or premandibular 

 pair of cavities are the only parts of the body-cavity within 

 the head which unite ventrally. In the trunk, however, the 

 primitively independent lateral halves of the body-cavity always 

 unite in this way. The section of the head-cavity just described 

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

 considered as equivalent to them. 



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

 may be called the mandibular cavity, presents during the stages 

 I and K a spatulate shape. It forms a flattened cavity, dilated 

 dorsally, and produced ventrally into a long thin process parallel 

 to the hyomandibular gill-cleft, PI. XIV. fig. 1 j)}). and fig. 7, 

 9 6 and 15 a, 2pp. Like the previous space it is lined by a 

 short columnar epithelium. 



The fifth nerve, as has already been mentioned, bifurcates 

 over its dorsal summit, and the mandibular branch of that 

 nerve passes down on its posterior and outer side. The man- 

 dibular aortic arch is situated close to its inner side, PL xiv. 

 fig. 7. Towards the close of this period the upper part of the 

 cavity atrophies. Its lower part also becomes much narrowed, 

 but its walls of columnar cells persist and lie close to one 

 another. 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. This thicker wall is 

 on its inner side separated from the surrounding tissue by a 

 small space lined by a membrane-like structure. In each of 

 the remaining arches there is a segment of the original body- 

 cavity fundamentally similar to that in the mandibular arch. 

 A dorsal dilated portion appears, however, to be present in the 

 third or hyoid section alone, and even there disappears by the 

 close of stage K. The cavities in the posterior parts of the head 

 become much reduced like those in its anterior part, though at 



