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- 
ine 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 (ig. 149). 
In the next four or five days two more appear, viz., the com- 
missura pallii anterior (tupffer), 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. Tur 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 
asomatic 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 
