No. 1.] THE DEVELOPMENT OF BALANOGLOSSUS. 55 
vesicle and in this way the dorsal blood-vessel is continued 
forward. 
Ventral to the notochord and at its sides, the remaining 
blastocoel space is filled with anastomosing mesenchyme 
cells. 
Larvae caught on June 24 and kept till July 3, show four 
pairs of gill-slits opening on the dorsal surface. Horizontal 
sections show best the structure of the gill region. Each gill- 
pouch extends ventrally below the lower level of its external 
opening. A sort of pocket formed in the endodermal wall, but 
Opening into the cavity of the digestive tract on one side, is 
found and into this hangs the tongue-bar, prolonged past the 
external opening ventrally into the pocket. Along a portion of 
the lateral walls of the pocket the endodermal walls are richly 
ciliated and over certain areas granular ciliated cells are found, 
as shown ir Fig. 74. 
Both in tne tongue-bars and in the dividing-bars or between- 
bars the chitin-like skeleton of the gill-region is laid down by 
the endodermal cells, as shown in Fig. 74. 
In each case the origin of these supporting-rods is a double 
one—arising from the anterior and posterior sides of their 
respective bars. Those in the tongue-bars are widely separated 
from the beginning and never unite subsequently, while the 
pairs formed in each between-bar are closely associated at the 
beginning and early unite into a single rod. 
In the upper dorsal corner of the gill region the rods unite 
inadefinite manner. The single rod in each between-bar is con- 
tinuous before and behind with a rod in the tongue-bars before 
and behind. Each tongue-bar is, therefore, held in position at 
its point of origin by a rod connected with a between-bar in 
front and with another behind. A series of inverted double 
U’s is found along each side of the body. 
The extension of the third body-cavities intc the tongue- 
bars is from above, z. ¢., at their point of origin, and the cavity in 
the bar separates the two rods from one another. 
The body-cavity found in the between-bars is a simple pro- 
trusion from the third body-cavities, continuous below with 
