4io 



HISTORY OF THE HUMAN BODY 



The material history of this advance appears to the mor- 

 phologist as the gradual modification of the simple neural tube 

 described above, a development which is traceable alike in the 

 comparison of adult animals, Class by Class, and in the em- 

 bryological record of a single animal, the lower forms preserv- 

 ing in greater detail the early stages of the history, the higher 



FIG. 115. Diagrams of the primary and secondary cerebral vesicles. 



(A) The primary vesicles. (B) The typical form of brain of vertebrates as de- 

 rived from A. 



The correspondence between the two is indicated by the horizontal dotted lines, 

 which mark off the areas of the primary vesicles, I, II, and III. 



forms recording the later stages. Completed in this way 

 from the numberless fragmentary records presented to the 

 investigator, the history of the neural tube in its progressive 

 modifications is as follows : 



It begins, so far as records go, with a form in which the an- 

 terior part of the tube, that corresponding to the head of the 



