446 ADAM SEDGWICK. 
mental duct precisely as does the pronephros in Amphibia. The 
only difference between the two is that in Teleostei the segmental 
duct has never more than one anterior opening, and the part of 
the body cavity into which it opens, and in which the glomerulus 
lies, is completely constricted off from the rest of the body cavity, 
and comes to resemble exactly an enormous Malpighian body.! 
I may here sum up the common features characterising the 
ontogeny of the pronephros and its duct (segmental duct) in 
all the forms of the Ichthyopsida in which the development is at 
all known: 
1. The segmental duct arises first as a ridge from the 
parietal peritoneum. ‘This ridge usually contains a diverticulum 
from the body cavity, and is continuously constricted off to form 
a duct.” 
2. Except anteriorly, where the constriction only takes place 
at intervals, leaving the openings of the pronephros (except in 
Teleostei, where there is only one opening). 
8. These openings correspond in number with the segments 
which the pronephros occupies.® 
4, A vascular structure, called glomerulus, is formed, pro- 
jecting on each side of the aorta into a specialised dilatation of 
the anterior part of the body cavity. Myxine forms a peculiar 
exception to this otherwise universal fact. 
5. This dilated part of the body cavity may become partially 
or completely separated off to form a capsule, into which the 
glomerulus projects and the anterior end of the segmental duct 
opens. 
j 6. The pronephros in all those Ichthyopsida in which it is 
found attaims a functional development, but is usually only 
active during a period intervening between the hatching and the 
attainment of full maturity, z.¢. it only functions in the larva. 
In Elasmobranchs, which do not, so far as is known, possess 
a pronephros, the segmental duct arises as a solid ridge from the 
somatic layer of the intermediate cell mass in the anterior region 
of the trunk. From this ridge there grows back a column of 
cells to the cloaca. On the development of a lumen the seg- 
mental duct, with its peritoneal opening, is established. The 
duct develops quite independently of adjacent structure behind 
1 There is a functional head-kidney in adult Ganoids. It appears to 
be formed on the Teleostean type (vide Balfour, ‘Comp., Embryology, 
vol. 2, p. 51). 
le Patotiyion, Scott (see note, p. 445) states that this duct arises as 
a solid rod of cells, which secondarily becomes connected with the body- 
cavity epithelium, to form the pronephric funnels. This account, in my 
opinion, needs confirmation. 
5 ¢Fiirb.,’ p. 5, p. 42. 
