180 SECTIONS DURING THE SECOND AND THIRD DAY. 



considered in connection with the development of the skull. 

 The last two disappear in the Chick without giving rise to any 

 permanent structures. The external opening of the first visceral 

 i.e. hyomandibular cleft becomes closed 1 , 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. 



Part of the membranous mandibular and hyoid arches form a wall round 

 the dorsal part of the original opening of this cleft, and so give rise to the 

 meatus auditorius externus. At the bottom of this is placed the tympanic 

 membrane, which is probably derived from the tissue which grows over the 

 dorsal part of the opening of the first cleft. It is formed of an external 

 epiblast epithelium, a middle layer of mesoblast, and an internal hypoblastic 

 epithelium. 



FIG. 116. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO 



WITH ABOUT TWENTY-FOUR MESOBLASTIC SOMITES. 



am. amnion ; so. somatopleure ; sp. splanchnopleure ; -wd. Wolffian duct ; st. seg- 

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

 cord ; c h. notochord ; ao. aorta ; hy. hypoblast. 



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

 Morphologisches Jahrbuch, Vol. in. 1877. 



