236 
G. L. PURSER. 
At Stage 36 (figs. 7 and 7a) the organ is clearly defined 
from the rest of the mesenchyme dorsal to the endodermal 
wall of the gut. It has sharply-marked boundaries, and is 
compact, while the neighbouring mesenchyme has developed 
the alveolar structure typical of ordinary connective-tissue. 
The capsule-forming cells appear as a single layer of 
nuclei bounding the organ. These cells have formed a 
definite connective-tissue sheath over a large portion of the 
surface by Stage 37, the most advanced stage I have 
examined. 
For the histogenesis of the spleen in Lepi do siren I 
cannot do better than refer to Dr. Bryce’s paper on the 
“ Histology of the Blood of the Larva of Lepidosiren.” 
One point only will I mention. The cellular elements show a 
marked reduction in size between Stages 35 and 37. This 
change seems to affect the whole of the mesoderm cells of 
the foregut, as will be seen by examining figs. 6, 7 and 8. 
I thought at first that it was due to variation in the amount 
of contraction which the larvae had undergone during the 
preparation of the sections, but that this is not the case is 
shown by two facts : (1) the blood corpuscles do not show 
this change, and (2) the effect is observable in all the series 
examined. 
To return to the question of the blood circulation. It is 
quite clear that this is, to begin with, entirely venous, so that 
there is a sort of splenic portal system. This is in connection 
with the veins draining the intestine. The development of 
these had not been fully worked out in either form, nor have 
I had the time to do it properly myself, so the following 
remarks must be accepted with all reserve. 
Apparently, in both species the main intestinal vein which 
drains the intestine breaks up in the spleen. (It will have 
been noticed that I have referred to the vein which supplies 
the spleen as the intestinal. There is, of course, no vein 
ordinarily called by that name : I use it simply as a matter of 
convenience, because I do not wish to make any definite 
statement on the point.) The blood from the spleen runs to the 
