SPLEEN OF LEP1DOSIREN AND PKOTOPTERUS. 
239 
Judging, therefore, from these three descriptions, taken 
with the facts of embryology already known, one comes to the 
conclusion that the original circulation of the spleen must 
have been entirely venous, being a portal system between the 
intestine and the liver. Later it was “ shunted ” oft the main 
vessel so as to lie on a loop alongside. Later still, the delivery 
of arterial blood removed the necessity of an afferent venous 
supply, so that in all forms above the Pisces there are present 
a single splenic artery and a single splenic vein only. In the 
class mentioned, however, both the veins persist, and there 
may be a second artery as well. The direction of the blood- 
flow in the veins is of some importance, a point on which 
authors are not very clear. Judging from T. J. Parker's 
description of Mustelus, it seems as if both the veins are 
efferent in function. This entails a reversal of the current, in 
the one serving the right lobe, during development. This is 
not a serious difficulty (it would be by no means an isolated 
case), but it makes the efferent system extraordinarily large 
compared with the afferent, both the veins being so much 
larger than the artery. 
It seems, therefore, as if detailed investigation of the blood- 
vessels of this part of the body in Lepidosiren and Pro- 
topterus would be of much morphological value, and would 
most likely help to bring into line the various descriptions 
which have been published for the different fish investigated. 
Summary. 
(1) The spleen arises in a thickening of the mesenchyme 
of the foregut, just after that mesenchyme has become free 
from yolk granules. 
(2) It is, at first, a mass of mesenchyme cells, round about 
which are comparatively large venous sinuses without any 
endothelial walls; later the cells become arranged to form 
trabeculae across these sinuses, which thus get broken up 
into the channels of a spongework. 
(3) The afferent and efferent veins are in very close con- 
