THE DEVELOPMENT OF VERTEBRATA. 275 



10. Sense-Organs. All, except the eye, and those con- 

 nected with the muscular and other internal senses, are 

 directly derived from the surface-epiblast. The most im- 

 portant are the olfactory and auditory organs. Each of 

 these starts as a simple pair of sacs of epiblast. The 

 olfactory sacs remain sacs in the dogfish, though their 

 walls undergo repeated folding. But in other types 

 they gain an internal opening into the mouth-cavity, and 

 are given a second function in connexion with respiration. 

 The internal ear is also a simple sac, and undergoes com- 

 plications in shape which lead to the formation of the 

 semicircular canals, etc. The endolymph duct persists 

 as the last trace of the original connexion with the 

 epiblast. It is a curious point that, in the frog, where 

 the epiblast is so early divided into epidermal and nervous 

 layers, only the latter sinks in to form the auditory sac, so 

 that this is never actually open at the surface, as in all other 

 types. 



The first visceral cleft is never actually perforate, but 

 it forms the middle ear (tympano-Eustachian recess) ; and 

 part of the hyoid arch (in frog and fowl) or of both hyoid 

 and mandibular arches (in rabbit) are pressed into the 

 service of the ear as auditory ossicles. (See chap, xiv., 7.) 



11. The Eye. The development of the eye in Verte- 

 brates is one of the most remarkable and at the same time 

 most instructive examples of a complex development. At 

 an early stago (second day in chick), while the cerebral 

 vesicles are still large thin-walled bulbs, a pair of hollow 

 outgrowths arise from the sides of the first vesicle and 

 extend towards the skin of each side of the head. These 

 are the rudiments of the retina, and their cavities may be 

 called the primary optic vesicles. They expand largely in 

 size, except close to the cerebral vesicle, their narrow con- 

 nexion with which may be called the optic stalk. Their 

 outer ends lie against the epiblast and become first flattened 

 and then greatly indented, so that the primary optic vesicle 

 becomes almost obliterated, and the whole structure assumes 

 something like the form of a goblet, but with its stem on 

 one side, and with a slit down that side of the bowl and 



