230 PRINCIPLES OF ANIMAL BIOLOGY 



This dorsal ganglion is in each nerve developed from one of the neural 

 crests. The dorsal root is completed by processes of nerve cells growing 

 inward from the neural crest and entering the dorsal part of the spinal 

 cord, and by other processes growing outward from the neural crest 

 toward the periphery of the bod}'' forming the sensory part of the spinal 

 nerve. The ventral motor root of the nerve grows out from the ventral 

 part of the spinal cord, and joins the fibers of the dorsal root 

 at a point bej^ond the ganglion. The nerve fibers from these two roots 

 remain distinct from one another, but are enclosed in the same connec- 

 tive tissue coverings. 



The development of some of the cranial nerves (those extending 

 from the brain) is in many respects similar to that of the spinal nerves. 



The Eye. — The principal sense organs are developed either as out- 

 growths from the central nervous system, or as ingrowths chiefly from 

 the ectoderm which come secondarily into connection with the nervous 

 system, or by a combination of these two modes of origin. The eye 

 begins as an evagination from the side of the brain (Fig. 182, ^). This 

 jirotnision elongates, and at the same time expands at its outer end into 

 a hollow bulb. The bulb-Uke expansion flattens on its outer side, and is 

 then invaginated to form a double walled cup resembling a gastrula 

 (Fig. 182, B and C). The inner layer of this cup becomes the visual part 

 of the retina and the basal stalk on which the cup rests is the optic nerve. 

 When the outgrowth from the brain comes in contact with the ectoderm, 

 the latter thickens and later invaginates, finally pinching off a rounded 

 mass of cells (B and C). This mass becomes the crystalline lens of the 

 eye. The ectoderm at the point where the lens was formed, becomes 

 transparent, and with additions from the mesoderm in most vertebrates 

 forms the cornea. The rest of the eye, including its muscles, is derived 

 from the mesoderm. 



The Ear. — ^The ear begins its development in the surface ectoderm, not, 

 as does the eye, from the central nervous system. A patch of ectoderm 

 on each side of the head region thickens, and then invaginates (Fig. 182, A), 

 producing a pear-shaped vesicle. The vesicle is pinched off from the ecto- 

 derm, and comes to lie within. It changes its shape, producing the 

 characteristic semi-circular canals and the (sometimes) coiled body of the 

 inner division of the ear. Only certain groups of cells in this vesicle 

 acquire sensory functions; these groups are joined to the brain by proc- 

 esses of nerve cells growing out from the ganglion of the eighth (auditory) 

 nerve which, as in the spinal nerves, is derived from one of the neural 

 crests. 



The middle ear, which contains the bones of the ear, is derived at 

 least in part from the first gill pouch (Fig. 182, A, B, C). In some 

 vertebrates this pouch never perforates to the exterior, in others it opens 

 for a time and then closes again. Its distal end comes to lie beside the 



