Janosik in 18871found, in a 3 mm. embryo of about 18 to 19 days, what he described as the pronephros or head kidney. It consisted of two rudimentary tubules, which were funnel shaped structures of peritoneum, and they extended from the mesothelial lining of the body cavity toward the primary excretory duct or the pronephric duct, into which one of the tubules opened. The pronephros is a vestigial structure having a brief existence, and probably no functional activity. It represents the earliest development of the excretory system. The mesonephros, or wolffian body, is developed as the intermediary organ of excretion between the time of degeneration of the pronephros and the appearance of the metanephros, or permanent kidney. The pronephros evolves the fundamental plan on which is based in a modified form the subsequent development of both the mesonephros and the metanephros. The paraxial mesoblast extends from the neural