AVES. 



149 



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

 directed somewhat downwards and their branches which slant up- 

 wards, the somewhat lozenge-shaped stomodseum is placed, 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 stomocfaeum or oral 

 cavity is called, from the parts which will be formed out of it, the 

 fronto-nasal process. 



In two succeeding pairs of visceral folds, which correspond with 

 the hyoid and first branchial arches of the Tchthyopsida, are de- 

 veloped the parts of the hyoid bone, which will be best considered in 

 connection with the development of the skull. The last two dis- 

 appear in the chick without giving rise to any permanent structures. 

 The external opening of the first visceral i.e. hyomandibular cleft 

 becomes closed', but the inner part of the cleft, opening into the 

 mouth, gives rise to the Eustachian tube and the tympanic cavity, 

 the latter being formed as a special diverticulum. 



'^F9 



sp c 



Fig. 116. Tkansyekse section through the trunk of a Duck Embryo with about 

 twenty-four mesoblastic somites. 

 am. amnion; so. somatopleure ; sp. splanchnopleure ; ivd. Wolffian duct; at. seg- 

 mental tube; ca.v. cardinal vein; vis. muscle-plate; sp.g. spinal ganglion; sp.c. spinal 

 cord; eft. notochord ; ao. aorta; /ly. hypoblast. 



1 Vide Moldenhauer, "Die Entwicklung des mittleren und des ausseren Ohres." 

 Morphologisches Jahrbuch, Vol. iii. 1877. 



