LATER DEVELOPMENT OF CHICK AND RABBIT 433 



The essential part of the ear then, like the eye, is derived from the 

 ectoderm. 



From the mesenchyme surrounding the auditory sac are developed, 

 first, a membranous covering for the labyrinth, with which it becomes 

 closely associated ; secondly, a loose tissue that becomes converted 

 into perilymph, and outside this a denser layer, in which the cartilage 

 and afterwards the bone is laid down. 



The hyomandibnlar pouch arises in two portions : a large ventral 

 part similar to the remaining gill pouches, and a smaller dorsal 

 portion which is perforated only for a short time. The ventral 

 portion is transitory, while the dorsal piece is persistent throughout 

 life as a part of the tympanic cavity. The remaining portion of the 

 cavity and the Eustachian tube are differentiated from the dorsal 

 wall of the pharynx in this region. The external auditory meatus 

 arises as an ectodermal ingrowth from the surface of the head in a 

 region that was originally between the dorsal and ventral portions of 

 the hyomandibular pouch. 



Olfactory Organ. 



The olfactory organs take their origin in much the same way 

 as the ear, as a pair of thickened patches of ectoderm, the olfactory 

 plates, lying on the side of the head in the fore-brain region in front 

 of the eye. They appear towards the close of the second day. The 

 plate invaginates and forms a fairly deep olfactory pit with a wide 

 opening, which, owing to a more rapid growth of the dorsal region of 

 the head, comes to lie on the ant ero- ventral side of the head. The 

 openings of the two pits are separated in the middle line by a broad 

 band of tissue, the fronto-nasal process, which thus forms their 

 inner margins as well as the anterior boundary of the primitive 

 mouth. The lateral wall of the aperture is elevated during the 

 fourth day to form the external nasal process. In the fifth day this 

 external process becomes linked with fronto-nasal process by a bridge 

 of tissue that separates the olfactory opening into parts, one above it 

 and one below. The bridge itself is a rudiment of part of the upper 

 jaw, and consequently as it enlarges the two apertures become 

 more and more widely separated ; the upper one moving dorsally 

 to form the external nans, and the lower one, lying in the stomodceal 

 area, passes into the buccal cavity to become the internal naris. 



Lepus. 



In the previous chapter, when considering the development of the 

 mammal, only the changes in the embryo itself were taken into 

 account, and it was indicated that while these were in progress others 



2 F 



