232 
O. L. PURSER. 
Just about this stage, however, the foregut elongates and 
narrows, and the mesoderm loses its granules, but remains 
fairly compact. 
On the right-hand side of the foregut, where it overlies the 
intestine, there is a distinct thickening of its layer of meso- 
derm, which is particularly vascular. This is where the 
spleen will arise (fig. 1 and la). It is in front of the dorsal 
rudiment of the pancreas, the only one which has developed 
at all at this stage, and, except for the posterior lobe of the 
liver, is entirely posterior to that organ. This posterior lobe 
is ventro-lateral on the right side of the position of the spleen, 
but more or less in the mesial plane of the embryo itself, the 
foregut being to the left of the middle line. 
The vascularisation is entirely venous, being part of the 
gut circulation. 
There is nothing to suggest that the endoderm has anything 
to do with the formation of this thickening, but it is impos- 
sible to be dogmatic on the point, because at stages just prior 
to the one under discussion the two layers seem completely 
fused, and it is impossible to decide in many instances whether 
a nucleus belongs to a cell of the endoderm or of the meso- 
derm. Since, however, all the organs known certainly to be 
of endodermal origin, e.g. liver, pancreas, or thyroid, arise 
as lieavily-yolked rudiments, therefore the development of 
the spleen from tissue entirely free from yolk granules and in 
all other respects resembling the mesenchyme of other parts 
of the embryo seem to lend support to the view upheld by 
Laguesse against that by Maurer and Kupffer. 
At Stage XXXIII (fig. 2 and 2a), the spleen rudiment is 
distinctly visible as a flat structure on the right side of the 
foregut extending as far forward as the point where the latter 
begins to bend to the mesial plane of the embryo over the 
lung outgrowth. The anterior ventral portion of the intestine 
extends for a considerable distance in front of the developing 
pyloric valve, and so the spleen rudiment is entirely dorsal to 
it. With regard to the glands of the alimentary canal, it lies 
behind the origin of the liver and the ventral pancreatic 
