396 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



fluid present in this cavity is known as the endo- 

 lyinph. 



In the mesoblast lying between these parts and the 

 cartilage, which at this period envelopes them, lymphatic 

 spaces become established, which are partially de- 

 veloped in the Sauropsida, but become in Mammals 

 very important structures. 



They consist in Mammals partly of a space sur- 

 rounding the utricle and saccule and called the vestibule, 

 into which open spaces surrounding the semicircular 

 canals, and partly of two very definite channels, which 

 largely embrace between them the cochlear canal. The 

 latter channels form the scala vestibuli on the upper side 

 of the cochlear canal and the scala tympani on the lower. 

 The scala vestibuli is in free communication with the 

 lymphatic cavity surrounding the utricle and saccule, 

 and opens at the apex of the cochlea into the scala tyrn- 

 pani. The latter ends blindly at the fenestra rotunda. 



The fluid contained in the two scalse, and in the 

 remaining lymphatic cavities of the auditory labyrinth, 

 is known as perilymph. 



The cavities just spoken of are formed by an absorp- 

 tion of parts of the embryonic mucous tissue between 

 the perichondrium and the walls of the membranous 

 labyrinth. 



The scala vestibuli is formed before the scala tympani, 

 and both scalse begin to be developed at the basal end 

 of the cochlea : the cavity of each is continually being 

 carried forwards towards the apex of the cochlear canal 

 by a progressive absorption of the mesoblast. At first 

 both scalse are somewhat narrow, but they soon increase 

 in size and distinctness. 



