252 THE DEVELOPMENT OF THE CHICK 



indicated. The surface is still smooth at the eighth day, but 

 on the tenth and eleventh days folds of the external surface 

 begin to extend into its substance, without, however, invaginat- 

 ing its entire thickness. These are the beginnings of the cere- 

 bellar fissures. 



The floor and ventro-lateral zones of the metencephalon enter 

 into the formation of the pons. 



In the roof of the isthmus, or constricted region between 

 cerebellum and mesencephalon, is found a small commissure 

 produced by decussation of the fibers of the trochlearis (Fig. 147). 



In the wall of the myelencephalon the neuromeres have dis- 

 appeared. The thin epithelial roof has become more expanded 

 in the anterior part (Figs. 147 and 148). Floor and sides have 

 become greatly thickened. 



Commissures. The brain commissures existing at eight days 

 are the anterior, posterior, inferior, and trochlearis (Fig. 149). 

 In the next four or five days two more appear, viz., the com- 

 missura pallii anterior (Kupffer), corresponding to the corpus 

 callosum of mammalia and the commissura habenularis. 



The development of the various nuclei and fiber tracts of 

 the bird's brain is entirely unknown and affords an interesting 

 topic for research. 



IV. The Peripheral Nervous System 



The peripheral nervous system comprises the nerves which 

 span between peripheral organs and the central nervous system. 

 There are fifty pairs in a chick embryo of eight days, of which 

 twelve innervate the head, and thirty-eight the trunk, distin- 

 guished respectively as cranial and spinal nerves. It is con- 

 venient for purposes of description to consider cranial and spinal 

 nerves separately, and to take up the spinal nerves first because 

 they are much more uniform in their mode of development 

 than the cranial nerves, and also exhibit a more primitive or 

 typical condition, on the basis of which the development of the 

 cranial nerves must be, in part, at least, explained. 



The Spinal Nerves. Each spinal nerve may be divided into 

 a somatic portion related primarily to the somatopleure and axis of 

 the embryo, and a splanchnic portion related primarily to the 

 splanchnopleure and its derivatives. In each of these again a 

 motor and sensory component may be distinguished. Thus each 



