DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 525 
anterior body cavity. This space is the first rudiment of the 
sac of the proboscis gland. Soon after its appearance it be- 
comes enclosed in a membrane, which is added first at the 
posterior part of the sac (cp. figs. 45, 31, and 47). Its cavity 
is therefore a tissue space arising in the wall of the body cavity, 
and it is in communication with the body cavity by means of 
the interstices between the cells bounding its anterior end. 
Its further development is involved with that of the heart, 
which had better be now described. The heart arises in ani- 
mals with three pairs of gill-slits, as a horizontal split in the 
tissue between the notochord and the sac of the proboscis 
gland. Its walls are very thin (v. fig. 52). From the first it 
appears to contain blood, which is apparently non-corpusculated, 
and can be coagulated by reagents. Whether the heart is 
originally in connection with the dorsal vessel or not could 
not be determined. Its walls soon become slightly muscular 
(v. figs. 67 and 97), and the pulsations, which can be dimly 
discerned through the skin in the living state, are doubtless 
occurring in this vesicle. 
After the formation of the heart a plexus of vessels in con- 
nection with it is formed among the mesoblastic cells covering 
the tip of the notochord (fig. 50). As this occurs the cells 
standing on the capillaries assume a pyriform shape, the sharp 
ends being fixed to the vessels and the wide ends free. These 
wide ends acquire a very transparent appearance, as though 
filled with fluid (fig. 49). These bunches of capillaries eventu- 
ally acquire a great development and communicate with two 
larger blood-vessels (fig. 53, b.v.), and with a sinus in the 
periphery of the gland. 
The sac of the proboscis gland anteriorly becomes filled up 
with a quantity of loose tissue, in which some granules of a 
yellowish colour are embedded. 
In B. minutus these yellow granules are of much com- 
moner occurrence (v. fig. 98). The capillaries of the gland are 
more regularly arranged. 
In B. salmoneus the capillaries are still more regular, 
running parallel to each other to the periphery of the gland, 
VOL. XXVI, PART 4,—NEW SER, MM 
