416 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



favour of the view that the anterior portion of the 

 brain was once separated from that which lies behind 

 it by the digestive tract. 



In no known Chordate, however, does the oeso- 

 phagus separate any one part of the nervous system 

 from the rest, and the whole mass is superior to or 

 dorsal in relation to it. In all Chordata also the 

 nerve cord has a central canal, and occupies exactly 

 the median axis of the body. The presence of this 

 canal is not to be explained without a reference to the 

 history of the development of the central nervous 

 system ; in this mode of development we find yet 

 another important characteristic of the Chordata. 



The median strip of epiblast which is to give rise 

 to the nerve cord, instead of merely sinking away from 

 the surface of the body, becomes grooved along its 

 middle line ; the sides of the groove grow up and 

 unite with one another, so as to leave a central cavity ; 

 in most cases the tube is first formed, and only later 

 on separates off from the layer of epiblastic cells 

 which forms the covering of the body ; in Amphioxus, 

 however, the external layer covers over the so called 

 " medullary plate " which forms the nerve cord before 

 the groove has become closed up. It will be seen 

 that, owing to the formation and closure of this 

 groove, the cells that were primitively external come 

 to lie within those that were primitively internal. 



In the Cephalocliordata the central nervous 

 system retains throughout life the form of a hollow 

 tube, and there is no distinct enlargement at the anterior 

 end which can be called a brain. In the U roeliordata 

 the typical arrangement is best seen in those which 

 retain the tail during the whole of their lives (Appen- 

 dicularia) ; in them we find an anterior swelling, 

 which becomes divided into two vesicles, with the 

 foremost of which an optic and an auditory organ 

 become connected ; the hinder vesicle is separated by 



