DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 99 
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, fuses 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 fibrous tissue in the base of the skin in the 
middle dorsal line (fig. 42). A similar strand (fig. 42) 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 larve with one to two gill-slits it has already 
