MINOT: VEINS OF THE WOLFFIAN BODIES IN THE FIG. 269 
immediately in contact with the epithelium, Ep, of the Wolffian 
tubules. Occasionally, as at much in the figure, a thin layer of 
mesenchyma intervenes between the vascular endothelium and the 
tubular epithelium, but the total amount of mesenchyma is insig¬ 
nificant. Almost everywhere the vascular endothelium is closely 
fitted against the tubules. It results that the mesonephros does not 
have a series of blood vessels in the ordinary sense, but is rather 
a single complex sinus, imperfectly subdivided by the Wolffian 
tubules. Hence every tubule is almost (or even for a certain part 
absolutely) completely bounded by a blood space, and accordingly 
bathed in blood from which it is parted only by the exceedingly 
thin endothelium. The circulation is therefore very unlike that 
through the capillary network of the pig’s kidney, for in the true 
kidney the capillaries are much smaller in caliber than the renal 
tubules, 1 and consequently only a small proportion of the epithelial 
excretory cells are in immediate proximity to the circulating blood. 
In the mesonephros of the pig, on the contrary, the great majority 
of the excretory cells are in actual contact with the attenuated 
vascular endothelium. 
In pig embryos, older than those of 12 mm., the Wolffian tubules 
are found increasing in length and (by budding) in number, but 
since the volume of the organ does not increase in the same measure, 
the tubules become more crowded, so that the intertubular sinus 
becomes more subdivided, and the vascular spaces of the sinus are 
reduced. Moreover the tubules occasionally become closely ap- 
pressed, and then the blood space between them may be obliterated. 
The general result of these changes is to cause the blood spaces to 
appear in sections less like parts of a great sinus, and more like 
separated vessels. An attentive study, however, of pigs of 14, 17, 
and 20 mm. shows conclusively that the circulation is only modified, 
and preserves the essential characteristics which it presents in pigs 
of 12.0 mm. 
The entire absence of capillaries and the sinus-like disposition of 
the blood channels characterize the circulation of blood between 
the tubules of the Wolffian body of the pig. This fact has already 
been confirmed, since the preliminary announcement of my results 
(Science, N. S., vol. 7, p. 229), by Parker and Tozier, 98 . 1 . My 
observations show that both characteristics recur equally well 
1 Except of course the small portion of Henle’s loop, which is almost as narrow as a 
capillary. 
