454 DR THOMAS H. BRYCE ON 
that is thus established. At no stage does the liver take any part in blood formation. 
The part taken by the spleen and the lymphoid tissue of the mesonephros and gut in 
the process will now be considered. 
Histogenesis of the Spleen. 
As already mentioned, the rudiment of the spleen appears in the splanchnic 
mesenchyme dorsal to the gut. It is simply a differentiated tract of that tissue. The 
hypoblast takes no direct share in its formation, but it must be observed that it takes 
form in the mesenchyme very shortly after that tissue is itself differentiated from the 
primitive undifferentiated mass of yolk cells. 
The first sign of its formation is the appearance of a column of large rounded or 
oval cells, round which the remaining cells of the mesenchyme group themselves con- 
centrically (fig. 28, Pl LV.). The nuclei of the investing cells are oval or elongated, 
and their long axes are arranged in a general way parallel to one another, and con- 
centrically to the central tract. As elsewhere, this tract of mesenchyme is permeated 
with spaces having no definite endothelial walls, but containing red blood corpuscles. 
These spaces are at first irregular, but in the second stage (fig. 29, Pl. IV.) they run 
together so as to form a peripheral smus surrounding the central tract, or island as it 
appears in sections, isolating it in great measure from the peripheral layers of investing 
cells. These latter become the investing connective-tissue coat of the spleen. The 
central tract is permeated by cleft-like spaces which communicate with the peripheral 
sinus. Both spaces and sinus contain red cells, and in the sinus are seen a number of 
_ leucocytes, evidently wandering in from the general mesenchyme. In the central mass, 
here and there are seen cells with simple nuclei surrounded by a layer of free protoplasm. 
Numerous mitotic figures occur among the cells of the central tract, which by multi- 
plication of its constituent cells increases in size, while at the same time its spaces are 
opened out until (fig. 30, Pl. IV.) it is converted into a system of cellular columns or 
trabecule. The peripheral sinus is now less definite in its arrangement, because the 
spaces between the outer ends of the columns have enlarged and become continuous 
with the lumen of the sinus. The peripheral investing cells are now all flattened 
connective-tissue elements, and the capsule of the organ is complete. 
The sinuses at this stage are full of red cells, nearly all of the mature variety. There 
are few erythroblasts, the phase contrasting in this respect markedly with the next, in 
which the sinuses are filled with young erythroblasts. 
The cellular columns are composed of cells of various dimensions (fig. 3, Pl. IV.). 
Some have smaller and irregular nuclei, but the great majority have round or oval 
nuclei of large size. In the spaces within the trabecule there are numerous white 
elements (fig. 31, Pl. IV.). These have either simple nuclei and hyaline protoplasm, 
or polymorphic nuclei and hyaline or granular protoplasm. The polymorphic nuclei 
have a closer arrangement of the chromatin nucleoli than the cells with the simple 
