WOLFFIAN DUCT AND BODY IN THE CHICK. 39 



with that with which in the succeeding segments it later unites 

 when the young segmental tubes acquire a communication with 

 the segmental duct. 



In Amphibia 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 developmejit 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. 38). 



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 e^g in developing those 

 organs which it will realU require as a larva, it leaves over the 



