390 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



The choroid slit closes very early, and is not per- 

 forated by any structure homologous with the pecten. 

 The only part of the slit which can be said to remain 

 open is that in which the optic nerve is involved ; in the 

 centre of the latter is situated the arteria centralis 

 retinae as explained above. From this artery there 

 grow out the vessels to supply the retina, which however 

 are distinct from the provisional vessels of the vitreous 

 humour just described, the blood being returned from 

 them by veins accompanying the arteries. On the 

 atrophy of the provisional vessels the whole of the blood 

 of the arteria centralis passes into the retina. 



Of the cornea, aqueous humour, eyelids and lacrymal 

 duct no mention need here be made, the account given in 

 Part I. being applicable equally to mammalian embryos. 



The auditory organ. In Mammals, as we have 

 seen to be the case in the chick (chap. VI.), the auditory 

 vesicle is at first nearly spherical, and is imbedded in ' 

 the mesoblast at the side of the hind-brain. It soon 

 becomes triangular in section, with the apex of the tri- 

 angle pointing inwards and downwards. This apex 

 gradually elongates to form the rudiment of the cochlear 

 canal and sacculus hemisphericus (Fig. 129, GG). At 

 the same time the recessus labyrinthi {R.h) becomes 

 distinctly marked, and the outer wall of the main body 

 of the vesicle grows out into two protuberances, which 

 form the rudiments of the vertical semicircular canals 

 {y.B). In the lower forms (Fig. 132) the cochlear 

 process hardly reaches a higher stage of development than 

 that found at this stage in Mammalia. 



The parts of the auditory labyrinth thus estabUshed 

 soon increase in distinctness (Fig. 130); the cochlear 



