DEVELOPMENT OP THE BRAIN. 963 



The Brain and Spinal Cord. 



The sides of the vertebral grocfve are lined on their surface by a 

 special layer of blastema, known as the medullary plates, derived from 

 the external germinal layer. When the groove and its cephalic ex- 

 pansion are closed, first in the neck, and then along the middle line 

 of the back of the embryo, a medullary canal with a cephalic enlarge- 

 ment is formed ; and the medullary plates, becoming thicker and 

 growing from below upwards, are converted, subsequently to the ap- 

 pearance of cartilages in the rudimentary vertebrae, into a tube of 

 primitive nervous substance, the anterior part of which is expanded 

 into three vesicles, placed at first one behind the other, but afterwards 

 bent with the head and neck of the embryo, and named the anterior, 

 middle and posterior, primary cerebral vesicles ; of these, the middle 

 one is much the largest. From them are developed, respectively, the 

 proxencephalon, the hinder part of which has been named the dien- 

 cephalon, the mesencephalon and the epencephalon, of which latter the 

 hinder part is called the metencephalon. The tubular portion of this 

 medullary canal forms the spinal cord, which at first consists of numer- 

 ous cells having a radiated arrangement around a central canal, and 

 for a long time retains its hollow condition. Even in the perfectly- 

 formed s-tate, it presents a rudiment of this cavity, in the so-called 

 central canal of the spinal cord (p. 249). The cells next to the 

 canal form its epithelial lining, or ependyma, whilst the outer ones 

 are developed into the nervous substance. The cord at first extends 

 throughout the entire vertebral canal, but afterwards it grows in 

 length less rapidly than the vertebral column, and the cauda equina 

 is gradually formed. The substance of the embryonic spinal cord is 

 composed of simple nucleated cells, which are developed chiefly 

 into the gray substance of the cord, but partly also into fine connec- 

 tive tissue and bloodvessels. The white substance of the cord is sub- 

 sequently formed. The peripheral part of the spinal nervous system, 

 as already mentioned, is developed, with the framework of the head, 

 trunk, and limbs, from the middle germinal layer. 



From the posterior cerebral vesicle, at first smaller, but soon larger 

 than the middle one, is evolved, in the metencephalon next to the 

 spinal cord, the medulla oblongata. At this point, the nervous sub- 

 stance, developed from the primitive medullary plates, does not form 

 a complete canal, as in the spinal cord, but remains open behind, con- 

 stituting the fourth ventricle, and is marked on its floor by the cala- 

 mus scriptorius, which leads into the canal of the spinal cord. Ante- 

 rior to the medulla, but still in the posterior cerebral vesicle or epen- 

 cephalon, appear the pons, and the rudimentary cerebellum, an angu- 

 lar projection forwards marking the line between them. At first, the 

 cerebellum consists of a thin transverse plate of nervous substance ; 

 then it enlarges, and becomes laminated, the central part, OF vermiform 

 process, is recognized before the lateral parts, or hemispheres; the 

 gray matter gradually becomes thicker on the surface, and the corpora 

 dentata are formed within ; the pons Varolii and the superior and in- 

 ferior peduncles also gradually enlarge. Owing to the bend which 



