194 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
embryos of 28.0 mm., Nos. 232, 233 and 234 of the Harvard Col¬ 
lection, similar vessels are present but their endothelium is in part 
already fitted against the tubular epithelium; the fitting is most 
marked when a vessel is between two coils of the tubule. In one 
embryo of 34.0 mm. (II. C. No. 202) the conditions are clearer, 
because the vascular spaces between the tubules have increased; it 
is now easy to trace frequent connections between these spaces and 
the cardinal veins, and also to observe the diminution of the mesen- 
chyma proper and the extension of the close adjustment of the vas¬ 
cular endothelium to the outer surfaces of the tubules. This 
adjustment is found in the dorsal region of the coils of the seg¬ 
mental organ, while towards the ventral side there is more mesen- 
chyma between the coils. As illustrating this condition, I will cite 
section 763 of this series, No. 202, but many other sections could, 
equally well, be chosen.“ Frontal sections of another embryo of the 
same length (H. Coll., No. 186, sections 76, 77, et al.) reveal the 
same condition. In an embryo of 37.0 mm., the most advanced at 
my disposal, the intertubular vascular spaces have increased, and 
both their free communications with the main channel of the car- 
Fig. 4. Bana virescens, mesonephros. Harv. 
Histol. Coll., No. 22. Transverse section. For 
clearness the nuclei of the vascular endothelium 
are drawn dark. 
dinal vein, and their 
intimate adaptation to 
the surfaces of the tub¬ 
ular epithelium, are 
more evident than in 
the embryos of 34.0 
mm., so that at this 
stage the circulation of 
the mesonephros may, 
I think, fairly be called 
sinusoidal. The condi¬ 
tions are well shown in 
the sagittal sections of 
this embryo, Harvard 
Embryological Collec¬ 
tion, No. 176, sections 
87, 88, et cd. Want of 
material prevents my 
carrying out these ob¬ 
servations further, but I 
hope to be able to pur- 
