EMBRYONIC DEVELOPMENT 



213 



crests. The dorsal root is completed bj'' 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 same cells in the 

 neural crest toward the periphery of the body, forming the afferent part of 

 the spinal nerve. The ventral root fibers grow out from the ventral part 

 of the spinal cord and join the fibers of the dorsal root at a point beyond 

 the ganglion. The nerve fibers from these two roots remain distinct from 

 one another but are enclosed in the same connective tissue coverings. 



Sense Organs. — The principal sense organs are developed either as 

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

 the ectoderm, which come secondarily into connection with the nervous 



Fig. 186. — Successive stages in the development of the lungs. The esophagus is 

 shown in A, B, and C, but not in D. As the lungs grow the mesoderm is pushed before them 

 and thus comes to invest the adult lungs and to make part of the lung tissue, br, bronchus; 

 es, esophagus; I, lung; m, mesoderm; tr, trachea. 



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

 begins as an evagination from the side of the brain (Fig. 184^). This 

 protrusion elongates and at the same time expands at its outer end into 

 a hollow bulb. The bulblike expansion flattens on its outer side and is 

 then invaginated to form a double-walled cup resembling a gastrula (Fig. 

 18-1:5, 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 near the ectoderm, the latter thickens 

 and later invaginates, finally pinching off a rounded mass of cells {B, 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 begins its development in the surface ectoderm, not, as does 



