574 THE EMBRYOLOGY OF THE FROG 



The neural crests are thus left between the central canal and the outer 

 ectoderm. 



The neural crests are quite large in the head region, becoming 

 smaller toward the posterior region of the embryo. Each crest becomes 

 divided into three masses as the neural plate begins to close. (Fig. 334.) 



The more anterior of these divisions which lies in the region of the 

 mid-brain is the beginning of the V nerve, and is to form the trigeminal 

 ganglion. The middle section is the beginning of the VIII and VII 



Fig. 335. 



The Nerve Placodes in the head of an Ammocoetes 4 mm. long. V 1 , first 

 ganglion of the V cranial nerve ; V a , second ganglion of the same nerve ; VII, 

 ganglion of the VII cranial nerve ; IX, ganglion of the IX nerve ; X, ganglion of 

 the X (Vagus) nerve. 1, 8, 13, first, eighth and thirteenth ganglia in the epi- 

 branchial series, for 1 , 6r 8 , first and eighth branchial pouches ; c, ciliary nerve ; eh, 

 notochord ; I, lateral ramus of the X cranial nerve ; N, milage of hypophysis ; n.a., 

 VI cranial nerve (abducens) ; o., ophthalmic nerve; r.f., recurrent branch of VII 

 cranial nerve (facial) ; r.v., recurrent branch of X (vagus) nerve; t., IV cranial 

 (trochlear) nerve; ves.op., optic vesicle. (From Vialleton after von Kupffer.) 



nerves, known also as the acustico- facialis ganglion, while the posterior 

 division forms the beginning of the IX and X nerves, or the glosso- 

 pharyngeal and vagus ganglia. The three divisions do not separate en- 

 tirely from the medullary plate, but remain connected by a very slender 

 chain of cells to the medullar region of the brain. 



When the tadpole has developed three or four somites, the inner or 

 nervous layer of the ectoderm opposite the crest ganglia proliferates to 

 form a patch, sometimes three or four cells in thickness. Such patches 

 are known as placodes (Figs. 268, 335), and are thought to be vestigial 

 sense organs. 



In the placode there is found a superficial sensory element which 

 may disappear, and a deep ganglionic element usually retained. It is 

 the ganglionic portion which fuses with the nearest crest-ganglion, thus 

 forming the principal sensory portions of the nerve. 



THE TRIGEMINAL OR V NERVE 



This is the principal nerve of the mouth and mandibular arch. The 

 trigeminal portion of the neural crest is large and extends from the eye 

 to the hyomandibular cleft (Fig. 335). The ectodermal and mesodermal 

 cell-groups fuse as the crest ganglion grows downward. In the ventral 

 region it meets the mesoderm of the mandibular arch. 



