THE HISTOLOGY OF THE BLOOD OF LARVA OF LEPIDOSIREN PARADOXA. 459 
appearances are much like those figured at stage 32 in the region of the mesonephros 
fie, 26, Pl. IIT.). , 
The tubules are imbedded in what is practically a huge sinus; in the blood stream 
a great variety of elements occur. There are numerous erythroblasts, many mono- 
nuclear cells, and numbers of leucocytes, while in the pseudo-lymphoid tissue there are 
large quantities of leucocytes of various kinds, but the polymorphic and granular 
varieties preponderate. The general impression when a section is compared with a 
section of the spleen is, that while in the latter the primary erythroblasts and secondary 
erythroblasts are greatly in the majority, the reverse is the case in the kidney. 
I have made some attempts to arrive at some estimation of the relative numbers of 
erythroblasts and leucocytes in the renal-portal and cardinal veins. As the vessels are, 
in the greater part of their course, unequal in size, any count, to be a reliable index of 
the part taken by this tissue round the kidney, in contributing new elements to the 
blood, would require to be one relative to the number of erythrocytes. This I found 
impracticable, on account of the corpuscles being unequally distributed. I therefore, to 
reduce the balance in a rough way, counted in every fourth section of a continuous 
series of 200 the erythroblasts and leucocytes in one renal-portal, and put them against 
those in the two cardinals. I have not sufficient confidence either in the method or 
the figures themselves to submit them in detail, or to found a definite judgment on 
them, but I may say that the general result was in favour of the renal-portal as regards 
both erythroblasts, large mononuclears, and leucocytes ; and that while erythroblasts and 
large mononuclears of the type seen issuing from the spleen pulp were very common 
in the renal-portal along the whole length of the kidney, they were practically absent 
in the cardinal. The possible explanation is, that in the ‘backwater’ formed by the 
great kidney venous sinus, the erythroblasts and their mother cells undergo their 
further transformation into erythrocytes. 
Thus, though I have shown that it is highly probable that both orders of corpuscles 
are produced along this tract at an earlier stage, it seems doubtful whether in the later 
phases the pseudo-lymphoid tissue of the kidney is concerned in the new formation 
of red cells. I have shown how the spleen, at first distinctly lymphoid, becomes 
later more specially concerned in the formation of the erythrocytes, and there is some 
reason for believing that the kidney tract is differentiated in the opposite sense. 
The development of this pseudo-lymphoid tissue, according to my account, is very 
simple. It is nothing more than the mesenchymatous tissue round the nephric duct 
and tubules, canalised, as it were, by venous spaces which communicate with the 
eardinal vein. From the first the cells are fixed and free; the fixed cells form the 
general connective-tissue basis; the free cells are either derived from the primitive 
mesenchyme cells of the tract itself, maintained by constant division, and set free in 
the blood stream by their own amceboid movement, or they wander to this site 
from the splanchnic mesenchyme. I have shown reasons for a belief in the first. 
alternative. . ; 
TRANS. ROY. SOC. EDIN., VOL. XLI. PART II. (NO. 19). 68 
