DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 85 
The partial transparency of the body wall permits also an 
indistinct view of the curious supporting rod of hypoblast 
which projects into the proboscis cavity. For reasons given 
later in this paper I propose to speak of this structure as the 
notochord (Nch.). The general appearance of the animal at 
the time when the second gill-slit appears, is shown in fig. 1. 
The animal is drawn as seen from the actual left side and dis- 
plays the increased flexion of the body on its ventral surface. 
The second gill-slit is shown as a small circular pore. 
It will also be observed that that part of the body which lay 
between the two grooves constituting the middle segment of 
the body has now assumed an altered shape. At Stage H is 
found little more than a circular ridge on the body separating 
the proboscis from the trunk, while in fig. 1 (two gill-slits) it 
forms a kind of phlange enveloping the base of the proboscis. 
This change in shape is due to the operation of several causes, 
which are of great importance in interpreting the processes by 
which the final form of the adult is reached. 
In the first place, after the formation of the mouth as a pore 
on the ventral surface, the constriction by which the proboscis 
is segmented off becomes deeper and deeper, until at last it is 
only attached by the exceedingly slender stalk shown in figs. 2 
and 3. As a consequence of this process coupled with a 
forward growth of the ventral lip of the collar, the mouth 
comes to be directed anteriorly instead of ventrally (cp. figs. 
7,46 and 57). By this process the anterior phlange of the 
collar acquires the relation shown in fig. 1, et seq. In 
addition to these changes a most important structure is first 
formed at this time, namely, the cavity, which from the rela- 
tions which it afterwards possesses I shall speak of as the 
atrial cavity. 
In the later conditions of Stage H the body is perceptibly 
wider in the region of the body immediately anterior to the 
gill-slits than it is behind them. This increase in width, which 
is still very slightly marked, is due to a circular thickening 
which passes all round the animal, being most developed at the 
sides. By the time of the appearance of the second pair of 
