164 THE DEVELOPMENT OF THE CHICK 
formed, but they rapidly become well differentiated. (See Chap. 
VIII.) 
IV. THE ORGANS OF SPECIAL SENSE (Eye, Ear, Nose) 
Kmbryologically a sharp distinction must be drawn between 
the essential percipient part of the organs of sense (retina of the 
eye, olfactory epithelium, and epithelium of the membranous laby- 
rinth) and the parts formed for protection and for the elaboration 
of function. The sensory part proper is the first to arise in the 
embryo, and is protected later by modifications of surrounding 
tissues or parts. We may thus distinguish between primary and 
secondary parts in the case of all organs of sense. Only the early 
history of the primary parts falls within the period covered by 
this chapter, except the formation of the lens in the case of the 
eye. 
The Eye. The primary optic vesicles arise, as we have seen, 
as lateral expansions of the anterior end of the neural tube; 
their position is indicated by an enlargement of the neural tube 
even before the meeting of the medullary folds in this region. 
The shape and relations of the early optic vesicles have already 
been described and figured. The origin of the optic stalk by 
constriction of the base of the vesicle was described in a preced- 
ing section of this chapter (p. 149). The stalks remain attached 
to the ventral end of the lateral walls of the diencephalon in 
the region of the recessus opticus, and constitute tubular con- 
nections between the vesicles and the brain, in the walls of which 
the optic nerve develops later (I*ig. 84). 
Locy found six pairs of “ accessory optic vesicles ’’ occurring in series 
immediately behind the true optic vesicles; they form low rounded 
swellings of the side-walls of the neural folds before the true brain 
vesicles are indicated, and last only about three hours in the chick 
(twenty-fourth to twenty-seventh hours of incubation). ‘‘ Their exist- 
ence supports the hypothesis that the vertebrate eyes are segmental, and 
that the ancestors of vertebrates were primitively multiple-eyed.”’ (Locy.) 
The external surface of the optic vesicle early reaches the 
ectoderm, to which it appears to be cemented at the 10s stage. 
In the 17-18 s stage, the optic vesicles project decidedly behind 
the attachment of the optic stalk, and the external wall is slightly 
thicker than that next the brain. The ectoderm then becomes 
thickened over a circular area in contact with the optic vesicle 
