242 T. H. BurLEN», 
tends over the posterior part of segment VII and the anterior 
part of segment VIII. The second tubule and its nephrostome 
atrophy. 
The vascular supply to the kidney may be observed on the 
right side only: 
3,52—3,69 aorta bulges out ventrally; 
3,54 the aortic out-growth fuses with the gut-wall; 
3.55—3,63 vitelline artery is cut longitudinally ; 
As \ the proximal end of the artery now seen in this section; 
3,66—3,67 a vascular outgrowth (glomus) of the vitelline artery 
projects into the coelom opposite the right pronephros; 
3,68, 69, 70 the vascular outgrowth fuses with somatic mesoderm 
in pronephric region and receives a branch from the aorta. 
(See Plate 25 Figs. 86—96.) 
A resumé of the development and degeneration of the pronephros 
in Seyllium canicula as indicated by my series of embryos A to K 
can be most conveniently given in tabular form (see page 243). 
From the table it will be seen that the pronephros in 
Scyllium canicula first appears at the time when 24 or 25 proto- 
vertebrae are present; the formation of the pronephric duct behind 
the pronephros can be traced back to the time when about 27 proto- 
vertebrae are present; the pronephros and its duct arise out of the 
some rudiment; the left pronephric duct grows back more quickly, 
at least for a time, than the duct on the right side; the pronephros 
on each side never extends over more than four segments, and be- 
comes limited very soon to three segments; the pronephric region 
comprises the metaotic segments VII, VIII, IX and X at first, and 
at a later period VII, VIII (and IX) only; nephrostomes become 
recognizable at a comparatively late period in the development; the 
nephrostomes, and the blood-vessels between, are not metameric; the 
blood-vessels on the left side of the body are undeveloped; the ne- 
phrostomes atrophy from behind forwards until one only on each 
side (in segment VII) persists as the coelomic opening of the 
Miillerian duct. The atrophy of the pronephric arteries is dealt 
with below. 
