THE ENTEROPNEDSTA. 19 



On the whole I am inclined to regard them as supporting 

 structures. 



The separation of the dorsal nervous system is much 

 more marked during this period than it was before it. It is 

 now completely separate from the point of junction of the pro- 

 boscis with the trunk to almost the level of the first pair of 

 gill-slits. At its anterior end (fig. 36) may be seen the be- 

 ginning of the process by which the anterior lumen is formed. 

 This is effected by a forward growth of the collar, together 

 with a continual sinking and horizontal invagination of the 

 nerve-cord. 



This lumen, thus formed, never extends for more than a 

 short distance into the cord, which, however, in its middle and 

 posterior regions in older animals, contains remarkable spaces 

 lined by columnar cells, more or less separated from each other 

 by strands of tissue, which will be described, together with the 

 later development and histology of the nervous system. 



The nerve-cord, as always, joins with the skin at both ends, 

 but from its posterior point of junction the rudiment of its 

 dorsal continuation in the skin may already be seen in section 

 as a small area of fi.brous tissue in the base of the skin in the 

 middle dorsal line (fig. 42). A similar strand (fig. 43) may 

 also be seen on the ventral side, beginning a little in front of 

 the first gill-slits. The two cords are still quite unconnected. 



The Proboscis Pore. — The first appearance of this struc- 

 ture is a thickening on the inner surface of the epiblast in the 

 proboscis stalk, which soon becomes hollow while still attached 

 to the skin (fig. 34, p. pr.). This epiblastic sac is from the 

 first asymmetrical, being on the dorso-lateral aspect of the 

 left side. From the first, the cells of which it is formed are 

 columnar, and it has no communication as yet (two gill-slits) 

 with the exterior or with the body cavity. 



Hypoblastic Structures. 

 Branchial Region and Notochord. — The process by 

 which the mouth comes to be forwardly directed has already 

 been described. In larvse with one to two gill-slits it has already 



