AVES. 



179 



from the fold on the other side, ,at a point in the middle line 

 nearer the front of the head than the junction of the main folds 

 (fig. 1 1 3, sm}. The two branches do not quite meet, being 

 separated by a median process, which at the same time grows 

 down from the extreme front of the head, and against which 

 they abut (fig. 120, /). Between the main folds, which are 

 directed somewhat downwards and their branches which slant 

 upwards the somewhat lozenge-shaped stomodseum is placed, 



M.c. 



So. 



So. 



FIG. 115. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN EMBRYO 



CHICK OF 45 HOURS. 



M.c . medullary canal ; P.v. mesoblastic somite ; W.d. Wolffian duct ; So. Somato- 

 pleure ; S.p. Splanchnopleure ; /./. pleuroperitoneal cavity ; ao. aorta ; v. blood- 

 vessels; iv. germinal wall; ch. notochord; op. junction between area opaca and area 

 pellucida. 



which, as the folds become more and more prominent, grows 

 deeper and deeper (fig. 120 A). The main folds form the 

 mandibular arch, and their branches the maxillary processes, 

 and the descending process which helps to complete the anterior 

 margin of the stomodseum or oral cavity is called, from the parts 

 which will be formed out of it, t\\Qfronto-nasal process. 



In two succeeding pairs of visceral folds, which correspond 

 with the hyoid and first branchial arches of the Ichthyopsida, 

 are developed the parts of the hyoid bone, which will be best 



12 2 



