THE DEVELOPMENT OF THE VERTEBRATE EXCRETORY SYSTEM
Authors: FRASER ELIZABETH A. 
Published: January 2008

I. The vertebrate excretory system is a holonephros. The entire organ arises from the one source, namely, from the intermediate cell‐mass which unites the somite with the lateral plate, and is originally segmental. The part of the intermediate cell‐mass adjacent to the somite develops into the nephrotome or nephric chamber, and the part connecting it with the body cavity becomes the peritoneal canal. The tubule is an outgrowth from the dorsal wall of the chamber and the duct arises from the union, in front and behind, of the distal extremities of the tubules and is therefore primitively segmental.

2. The simplest relations appear to be found in myxinoids so far as can be ascertained from the somewhat incomplete knowledge we possess of this group. They are also primitive in Gymnophiona and detailed reference has therefore been made to this group.

3. The original terms pronephros, mesonephros and metanephros are retained. The term pronephros should only be applied to the organ in larval Anamnia and to that of a few adult teleosts. In most vertebrates, behind the pronephros where this is present, or posterior to the zone of anterior rudimentary tubules, the intermediate cell‐mass is early separated from somite and lateral plate and forms the so‐called nephrogenic blastema from which the remainder of the excretory organ is derived. The later differentiation is one of size and complexity only. The gradual separation of the hinder portion into a well‐defined isolated metanephros, such as is present in the Amniota, is indicated in some elasmobranchs and in some urodeles, and is seen in various species of teleosts.

4. Recent experimental work with Amphibia has located the area from which the organ develops in the gastrula. The methods of pronephrotomy and transplantation have substantiated the earlier observations as to the precise level at which the pronephros arises and that from which the duct takes its origin. The capacity of the duct to grow independently backwards towards the cloaca has been conclusively proved by the use of vital staining. By means of bisection, partial removal and transplantation an attempt has been made to ascertain the power of regeneration of the duct and also the interrelations between the latter and the developing mesonephric blastema. Interesting information has been acquired with regard to the role of induction in these interrelations.

5. Attention is drawn to the distinction between the ciliated nephrostomial canal and the ciliated peritoneal canal; their initial derivation and later development in normal and abnormal conditions is discussed.

6. The relationship between external and internal glomeruli is considered, and the passage from one to the other as met with at the anterior end of the mesonephros in many Amniota is described. It is emphasized that the function of such a condition is by no means clear and needs experimental investigation.

7. It is concluded that the homogeneity of the pro‐, meso‐ and metanephros has now been convincingly established from many points of view. The nephrogenic material arises from a single source and exhibits identical properties throughout, and there is striking uniformity in the structure and function of the various parts of the system.