246 



Mr. A. S. Packard 07i the Aspects of the 



blastopore to the anterior extremity of tlie head, and known 

 as the meduHarj plate, becomes isolated from the remainder 

 of the layer to give rise to the central nervous axis ;" in Tuni- 

 cates as well as Vertebrates this plate is converted into a tube 

 or canal, which lies wholly above the alimentary tract. It is 

 this striking feature in embryo Tunicates which mainly seems 



Ym. 2. 



EmLryo of an Ascidian, showing the vertebrate plan of structure ; the 

 nervous system Ui,h) Avith the spinal nerves (s) being situated dor- 

 sally abo^e the notocliord (c) and alimentary canal {b,i). 



to justify their elimination from the Worms and indicates their 

 proximity to the Vertebrates, as this seems to be a more truly 

 vertebrate feature than even the possession of a notocliord. 



Balfour states on p. 342 : — " The spinal cord, shortly after 

 the closure of the medullary canal, has, in all the true V^erte- 



Fig. 3. 



Section of a vertebrate embryo (a iish) : n, nervous tube, open in front 

 and situated dorsally ; oh, notcchord j hh, mouth ; e, alimentary 

 canal; a, place of veut ; m, mesoderm. 



brata, the form of an oval tube, the walls of which are of a 

 fairly uniform thickness, and are composed of several rows of 

 elongated cells. This cord, as development proceeds, usually 



