TAIL-EEGIOX OF POLISTUTKEMA (bDELLOSTOMa) STOUTI. 843 
Yl . Formation of the Caudal Heart Valves. 
As lias already been pointed out in the 27 mm. series there 
is a slig*ht massing of the mesenchyme {val. (i.) around the 
orifice of the first or anterior communication between the left 
ventral veno-lymphatic trunk and the left caudal heart, which 
may be the forerunner of two valves that will occur later. 
It is not, however, until the 58 and 60 mm. series are reached 
that any real clue is obtained as to the manner of the forma- 
tion of these valves. 
In the 60 mm. series the right and leftvalves (Fig. 16, Val. (i^) 
guarding the orifice of the first or anterior communication 
between the left ventral veno-lymphatic trunk and the left 
caudal heart are conspicuous and fairly well formed. They 
are composed of masses of mesenchyme or of but little differ- 
entiated connective tissue, which has’ migrated dorsad and 
inward into the heart from the right and left walls of the 
ventral trunk’s connecting vessel. A close inspection of their 
structure will disclose that, notwithstanding* that they belong 
to the same outer connective-tissue layer of the heart and 
ventral connecting vessel, yet their differentiation into con- 
nective tissue has not advanced nearly as far. The valves 
guarding the second or posterior communication of the ventral 
trunk with the left heart are in a much earlier stage of 
formation, in fact, only the very beginnings of embryonic 
connective-tissue valve-folds are visible on either side of the 
orifice. What appears to be an intermediate stage is shown 
at the orifice of the first or anterior communication of the 
ventral trunk with the right caudal heart. Here the right and 
left valve-folds of but little differentiated connective tissue 
from the side walls of the communicating vessel have grown 
inward and dorsad, and have fused irregularly, so that in 
places there would be a solid wall of tissue, and in other 
places temporary openings. Fig. 17 shows a region of these 
valve-folds where there is an opening on the left side, while 
in the next section cephalad (Fig. 18) there has been a com- 
plete fusion of the valve-folds, so that there is no orifice, and 
VOL. 59, PART 2. — new series. 22 § 
