1. The entire mesonephros of R. temporaria is derived from the mesonephric blastema, a mass of cells originally occupying a position along the dorso-medial wall of the archinephric duct and later along the dorso-medial angle of the kidney.
2. The mesonephros arises as two perfectly distinct sets of units which are termed the ‘early units’ and the ‘later units’.
3. The early malpighian units are derived as from small spherical condensations (termed ‘nephroblast vesicles’) in the blastema.
4. Each of these nephroblast vesicles elongates into a tubule which forms a connexion with the archinephric duct at one end and develops a malpighian capsule at the other.
5. These early units later sever their connexion with the archinephric duct and degenerate.
6. The early peritoneal funnels are derived from masses of blastema lying between the early malpighian capsules and the periphery of the mesonephros.
7. The lumina of these funnels never form any connexion with the lumina of the malpighian capsules, but form a direct connexion between the coelom and the blood-system.
8. The growth and coiling of the early units forces the archinephric duct away from the blastema into the dorso-lateral angle of the kidney.
9. During the course of this separation a special set of condensations arise.
10. These condensations elongate into straight tubules which maintain a connexion between the archinephric duct (into which they open) and the blastema.
11. That end of the straight tubule which lies in the blastema develops an abortive malpighian capsule at its tip.
12. The later malpighian units arise as condensations (‘capsuloblast vesicles’) in the blastema when this latter has become separated from the archinephric duct.
13. The capsuloblast vesicles are not formed singly but in vertically hanging strings; this is due to the fact that as each vesicle develops it is pushed downwards by the growth of the blastema above it, while a further vesicle condenses in the clear patch of blastema so left.
14. The capsuloblast vesicle differentiates into capsule and glomerulus, from the former of which a tubule grows out.
15. This tubule forms a connexion with a bud which has grown out from one of the straight tubules.
16. When the most ventral capsule of each string approaches the peritoneal wall of the kidney, it is seen to be separated from the latter by a small mass of blastema.
17. This mass of blastema develops into a peritoneal funnel exactly as do the blastema masses lying between the early malpighian capsules and the peritoneum.
18. A further set of condensations (the vesicles of the ‘funnel-forming tubule’) now arise in the blastema.
19. Each of these vesicles elongates into a tubule which runs down towards the peritoneal wall of the kidney.
20. The tubule so formed becomes internally ciliated along the lower third of its length.
21. The tip of one of these tubules enters one of the peripheral blood-vessels and places itself parallel to the outer wall of the mesonephros.
22. The whole tip of the tubule now breaks off and, by acquiring a connexion with the coelom, becomes a perfect peritoneal funnel.
23. It is suggested that the ‘early units’ represent an ancestral mesonephros and that the later units are homologous with the whole of the Urodelan mesonephros as it is at present known.