Abstract
The differentiation of sex in the mouse is accompanied by an increase in the diam. of the testis over the ovary, detectable on the 12th day. The mesonephros is small and non-functional, and is made of 12 tubules. Glomeruli are absent; Bowman''s capsules are represented by slight swellings of the blind ends of the tubules. The 8 anterior tubules are connected with the peritoneum by nephrostomes. The rete testis is formed by strands derived from 6 to 8 nephrostomal canals. The corresponding tubules form the epididymis and epoophoron, the rest degenerate.