138 OUTLINES OF CHORDATE DEVELOPMENT 



and when the neural tube is fully formed, each may be seen to 

 be' connected with the dorsal surface of the myelencephalon 

 by a slender chain of cells (Fig. 45). 



Opposite each of these crest ganglia the cells of the inner or 

 nervous layer of the ectoderm very early (stage with 3-4 

 somites) proliferate and form a patch, in places three or four 

 cells deep (Fig. 45). These thickened patches are known as 

 placodes: they are undoubtedly to be interpreted as vestigial 

 sense organs. In each placode two separate elements are 

 distinguishable, a superficial sensory element, which usually 

 disappears (the exceptions will be noted below), and a deeper 

 ganglionic portion which is usually retained to a varying extent. 

 The ganglionic portion of the placode typically fuses with the 

 cells of the associated crest ganglion, and together they form 

 the rudiment of the chief sensory or afferent components of 

 the cranial nerve. From this point onward we may describe 

 separately the history of the chief cranial nerves. (Reference 

 is unavoidably made to the visceral arches and clefts whose de- 

 velopment must be described later, in connection with the 

 history of the pharynx). 



The Trigeminal Nerve (V). This is the chief nerve of the 

 mouth and mandibular arch. The trigeminal portion of the 

 neural crest is very large, extending from the eye to the hyo- 

 mandibular cleft (Fig. 44, C, E, F). The crest ganglion grows 

 downward and comes ventrally into contact with the mesoderm 

 of the mandibular arch. These ectodermal and mesodermal 

 cell groups then fuse, cells of the apposed surfaces intermingling, 

 and finally a cell mass is formed in which the two elements are 

 indistinguishable; this becomes the mesenchyme of the man- 

 dibular arch (Fig. 45, A, B). 



The dorsal and superficial cells of the crest ganglion retain 

 their nervous character and come into relation with the large 

 placode of this region. The superficial or sensory portion of 

 this placode disappears, but its deeper or ganglionic portion 

 enlarges and divides into two parts. An anterior part separates 

 from the surface ectoderm as the ophthalmic ganglion of the 

 ophthalmic branch of the V nerve, whose fibers grow out ante- 



