Il8 EARLY EMBRYOLOGY OF THE (HICK 



no differentiation in four-day chicks. Later in development 

 there is ventrally and laterally an extensive ingrowth of fiber 

 tracts giving rise to the pons and to the cerebellar peduncles 

 of the adult metencephalon. The roof of the metencephalon 

 undergoes extensive enlargement and becomes the cerebellum 

 of the adult brain. 



The Myelencephalon. In the myelencephalon the dorsal 

 wall has become greatly reduced in thickness indicative of its 

 final fate as the thin roof of the medulla. Like the roof of the 

 diencephalon, the roof of the myelencephalon later receives a 

 rich supply of small blood vessels by which it is pushed into 

 the myeloccele to form the posterior choroid plexus (choroid 

 plexus of the fourth ventricle). The ventral and lateral walls 

 of the myelencephalon become the floor and side-walls of the 

 medulla of the adult brain. 



The Ganglia of the Cranial Nerves. In the brain region, cells 

 derived from the cephalic portion of the neural crest have be- 

 come aggregated to form ganglia. The largest and the most 

 clearly defined of the ganglia present in four-day chicks is the 

 Gasserian ganglion of the fifth (trigeminal) cranial nerve (Fig. 

 42, B). It lies ventro-laterally, opposite the most anterior 

 neuromere of the myelencephalon. From its cells sensory 

 nerve fibers grow mesiad into the brain and distad to the face 

 and mouth region. In four-day chicks the beginning of the 

 ophthalmic division of the fifth nerve extends from the ganglion 

 toward the eye, and the beginning of the mandibulo-maxillary 

 division is growing toward the angle of the mouth (Fig. 40). 

 Immediately cephalic to the auditory vesicle is a mass of neural 

 crest cells which is the primordium of the ganglia of the seventh 

 and eighth nerves. The separation of this double primordium 

 to form the geniculate ganglion of the seventh nerve and the 

 acoustic ganglion of the eighth nerve begins during the fourth 

 day. Posterior to the auditory vesicle the ganglion of the 

 ninth nerve can be clearly seen even in whole-mounts (Fig. 40) . 

 The ganglia of the tenth (vagus) nerves can be recognized in 

 .sections of chicks at the end of the fourth day but are difficult 

 to make out in whole-mounts. 



The Spinal Cord. The spinal cord region of the neural tube 

 when first established, exhibits a lumen which is elliptical in 

 cross section. As development progresses the lateral walls of 



