34 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER III. 

 DEVELOPMENT OF THE NERVOUS SYSTEM. 



It is hoped in this chapter to bring to the attention of the student 

 those features and processes in the development of the nerv^ous 

 system which are of the greatest morphological importance, and 

 especially to bring forward certain facts which have not received full 

 treatment in text-books of embryology. There will not be space 

 for such a full and orderly treatment of the development as an 

 elementary student in embryology would require. For this reason 

 it will be assumed that the student is familiar with the more 

 general facts in the development of some vertebrate such as the 

 frog or chick. 



It is commonly believed that all nerv^ous functions are performed 

 by structures derived from the ectoderm. In the early vertebrate 

 embryo a certain area on the dorsal surface is to be recognized 

 as the anlage of the greater part of the nervous system. This 

 area is known as the neural plate. It becomes visible first as an 

 area boimded on either side by sUghtly elevated ridges or folds, 

 the neural folds. These folds are continuous in front and behind 

 and enclose an area which is broader in front, in the region corre- 

 sponding to the future brain (Fig. i8 A). This somewhat banjo- 

 shaped neural plate and the folds which bound it give rise to the 

 brain and spinal cord, the greater part of the sensory nerves with 

 their gangHa, the motor nerves and the retina. The olfactory 

 epithelium, the sensory epithelium of the ear and lateral line 

 organs, and the gangha and ner^^es of these organs are derived 

 from parts of the ectoderm closely related to the neural plate. 

 The organs of taste appear first in the embryo in the entodermal 

 lining of the branchial cavities. In fishes they appear later in 

 the outer skin also and their history requires further investigation. 



In all vertebrates the neural plate becomes converted into a 

 tube lying below the surface of the body. In all except cyclostomes 



