430 DEVELOPMENT OF ELASMOBRANCH FISHES. 



is situated close to the eye, and presents a rounded or sometimes 

 triangular figure in sections (PI. 15, figs. 7, 9 b and i6b, I pp.}- 

 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. 15, fig. 8, i //.). 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 con- 

 sidered 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, PL 15, fig. I //. and fig. 7, 9 b 

 and 1 5 a, 2 //. 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 mandibular 

 aortic arch is situated close to its inner side, PI. 15, fig. 7. To- 

 wards 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 splanch- 

 nic 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 



