SPLEEN OF LEPIDOSIREN AND PROTOPTERUS. 
233 
buds but anterior to the dorsal bud, a position it keeps 
throughout life. 
As yet the cells of the organ are quite undifferentiated 
from those of the rest of the mesenchyme, but the rudiment 
is apparent owing to its vascularisation. The vein from the 
intestine here breaks up into a number of branches which 
run into the rudiment in a more or less forward and dorsal 
direction. This is the afferent system. The efferent is formed 
by a number of small tributaries which unite and enter the 
liver as the Hepatic Portal Vein. The organ itself contains 
large sinuses, chiefly peripheral, with which these two 
systems communicate. They have no visible endothelial 
lining. 
About this time in the development of the embryo the 
foregut greatly increases in length, and since the spleen lies 
embedded in its sheath it is to be expected that it would 
grow in that direction too. Examination of embryos at 
Stage XXXIV proves this to be the case, its length increasing 
from about; *2 mm. at Stage XXXIII to about *5 mm., while its 
greatest diameter remains at about T5 mm. Anteriorly it is 
slightly twisted towards the dorsal side of the foregut, where 
the latter begins to arch over the lung rudiment : which seems 
to point to this portion of the foregut being at this stage 
included in the twisting which occurs chiefly in the intestine. 
The position of the spleen with regard to the gut appendages 
is the same as earlier, i.e., the bile-duct opening and the 
ventral pancreatic buds lie roughly in tho same transverse 
plane as its anterior end, and the dorsal pancreas lies at the 
posterior end. The yenous circulation of this portion of 
the embryo has undergone no change, except, perhaps, that 
the peripheral sinuses are better marked, and I have been 
unable to trace any arterial supply. 
There is active cell-division going on throughout the organ, 
but particularly in the venous spaces, where the erythroblasts 
are multiplying freely. 
At this stage (fig. 3 and 3a) the differentiation between the 
cells of the spleen and those of the rest of the mesenchyme 
