196 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
nephrotome early becomes S-shaped and unites laterad with the 
Wolffian duct, thus becoming a Wolffian tubule, which lies in and is 
completely surrounded by embryonic mesenchyma, see Fig. 134, p. 
238, of my “Human Embryology.” In a chick of 96 hours the 
conditions resemble somewhat those of an Acanthias embryo of 28 
mm., for the tubules are coiled and blood spaces are present 
between the coils, but there is also still mesenchymal tissue between 
the coils ; the intertubular blood spaces communicate directly with 
the adjacent cardinal vein, and their endothelium is in part fitted 
close against the mesonephric epithelium. 1 have examined three 
embryos of this stage (Harvard Embryological Collection, Nos. 98, 
99, 100). In a chick of six days (II. Coll. No. 254, slide B) the 
mesonephros is much developed, and although some mesenchyma 
is still conspicuous between the tubules, yet the blood spaces have 
enlarged into characteristic sinusoids, the endothelium of which 
adheres for most of its extent very intimately to the tubules and 
only here and there extends over the surface of the patches of 
mesenchyma. Finally in a chick of alleged ten days incubation, 
but apparently really older, I find the condition represented in 
Fig. 5. There is a layer of mesenchyma entering into the forma¬ 
tion of the sub-mesotlielial capsule of the organ, and there is some 
mesenchyma at the bases of the Malpighian corpuscles, but between 
the tubules I cannot make sure of there being mesenchyma left, save 
the cells of the vascular endothelium. As to how the mesenchymal 
cells disappear, I have not been able to satisfy myself; there is no 
evidence of histolysis, so that one cannot avoid the surmise that all 
the mesenchymal cells are utilized to form the vascular endothelium. 
The method of development of the mesonephric sinusoids in the 
pig and the rabbit is similar to that in the chick. In these three 
types, as in Acanthias, so far as studied, there are no true capillaries 
in the Wolffian body at any stage. The segmental tubules are at 
first short and curved; as they lengthen they become coiled and as 
the coils increase they lie close together, so that there is space left 
only for a very limited amount of mesenchyma between them; after 
the coils have begun, and while their formation is continuing, blood 
spaces first appear around and between the tubules; 1 the spaces 
communicate freely with the adjacent cardinal vein; the blood 
1 This stage may be observed in rabbit embryos of 12^ days, while in pig embryos of 6.0 
mm., the youngest of which I have sections, the intercrescence of vein and tubules is 
already well advanced. 
