CHAPTEK XTX. 



DEVELOPMENT OF THE HUMAN BRAIN DURING UTERINE TilFE 



In the long axis of the clear ' germinal area' of the human 

 impregnated Ovum, there appears an opaque line of young 

 tissue known as the ' chorda dorsalis.' 



Above this, and along its entire extent, a shallow 

 'primitive groove' is found, which soon becomes bounded 

 on each side by an up-growing lamina of new embryonic 

 tissue. These laminas gradually approach one another, 

 and ultimately unite over the before-mentioned ' primitive 

 groove' so as to form a distinct tube closed at each end. 



The internal layer of this tube grows in thickness so 

 that its bore becomes gradually narrower. It soon differ- 

 entiates also into two distinct textures. The most internal 

 of these, viz., that immediately surrounding the diminished 

 central canal, is composed of embryonic nerve tissue, and 

 is destined to develop into the Cerebro- Spinal Axis. 



The diameter of this hollow, rudimentary nervous axis 

 is not uniform throughout its whole extent. Even before 

 the laminae completely close over the * primitive groove,' 

 the anterior extremity of the embryo tube swells into 

 three dilatations immediately contiguous to one another ; 

 and it is from the nerve tissue in these swellings, together 

 with that of certain important outgrowths therefrom, that 

 the several parts of the Human Brain arc developed. 

 From the portion of the tube behind the three swellings 

 the Spinal Cord is formed. 



