458 ADAM SEDGWICK. 
with that with which in the succeeding segments it later unites 
when the young segmental tubes acquire a communication with 
the segmental duct. 
In Awphibia the segmental duct, when larval life is tolerably 
advanced, opens into a Wolffian tubule, which arises from a mass 
of cells, the origin of which is obscure, but which apparently 
does not appear till after the larva has left the egg. Now the 
Wolffian tubule of an Amphibian is homologous with that of an 
Elasmobranch ; it is similarly constructed, and opens into the 
body cavity at a corresponding point. Hence we are driven to 
the conclusion that the cells from which the Wolffian tubule in 
an Amphibian arise are homologous with the intermediate cell 
mass of an Elasmobranch. 
But in Amphibia these cells are not developed where, if 
Elasmobranch development is primitive, they should be; and 
appear later in a way which gives no clue to their relationship 
to the intermediate cell mass in Elasmobranchii. 
What is the meaning of this extraordinary method of deve- 
lopment ? 
In Elasmobranchs the development of the segmental duct is 
modified, while the development of the mesonephros is primitive 
in its segmental arrangement and origin as a specialised part of 
an organ present at an earlier stage. 
In Amphibia the development of the segmental duct is more 
primitive, but that of the mesonephros very modified, and this 
very latter fact always goes hand in hand with the presence of a 
pronephros. Turning to the pronephros, it is found to develop 
in continuity with the segmental duct. It is found to possess, 
with regard to its openings into the body cavity, a segmented 
structure. It is also found to possess a structure, the glome- 
rulus, resembling extraordinarily closely the glomerulus of an 
ordinary Malpighian body of the mesonephros. ‘This glome- 
rulus lies in a special part of the body cavity, just as a glome- 
rulus of a Malpighian body in the mesonephros of an 
Elasmobranch lies in what from its origin may be called a 
specialised part of the body cavity; and both these specialised 
sections in their anatomical position precisely correspond (see 
above, p. 457). 
With all these similarities can the inference be avoided that 
the head-kidney is descended from the same primitive excretory 
system as the mesonephros, which has appeared early in develop- 
ment to supply the larva with an excretory organ, and has been 
able to retain a more primitive development ? The larva, having 
this, has not wanted the hinder part, and in consequence, having 
all its energy occupied while within the egg in developing those 
organs which it will really require as a larva, it leaves over the 
