TAIL-REGION OF POLISTOTREMA (bDELLOSTO.Ma) STOCTI. 321 
•of blood in the lymphatics. But how account for the great 
number of red corpuscles found in embryos before any muscle- 
fibres were developed, and for older embryos and adults where 
a great proportion of the corpuscles were red ? It has already 
been stated that no direct corrections have been found 
between the arteries and the veno-lymphatics ; that the veno- 
lymphatics of the adult, especially the lateral sinuses, contain 
very few corpuscles compared to the late embryos, although 
the red greatly predominate ; that there are a multitude of 
red corpuscles apparently being formed in the connective 
tissue surrounding the lateral siuuses of the late embryos. 
Is it not, therefore, more than likely that in embryos the 
veno-lymphatics, especially the lateral sinuses, function as 
reservoirs or receptacles to hold the blood-corpuscles germi- 
nated in the nearby connective-tissue mesenchyme, and that 
the corpuscles migrate through the loose endothelial cells of 
the lateral sinuses, where many of them are seen undergoing- 
cell division ? 
There can be no question but that the dorsal, lateral and 
ventral veno-lymphatics and the caudal hearts of Polisto- 
trema are the homologue of similar structures found in 
Teleosts, Ganoids and Selachians. 
III. Late Embryonic Condition of the Blood-Vessels. 
In all of the good series, which began with two 20 mm. 
■embryos, the caudal artery and vein were well formed, hence 
onlv a o-uess can be made at the manner of their formation. 
%/ O 
In the 27 mm. series, which was carried further cephalad 
than the others, I found in a section taken through the hind 
gut that the caudal vein was in direct connection with the 
splanchnic blood-vessels of the yolk-sac. This may indicate 
a similar origin of the two, and from my point of view would 
expect the splanchnic vessels of the yolk-sac to arise as they 
do in the chick blastoderm by the union of certain blood 
islands or mesenchymal spaces, and the transformation of the 
border mesenchyme into endothelium and the enclosed cells 
into red corpuscles. 
