ABSTRACT
The pronephric anlage is indicated as a slight thickening of the somatopleural portion of the lateral mesoderm at stage 21. The posterior part of the pronephros is unorganized at stage 28. At this stage the first nephrostome and its canaliculus appear in the form of a cell concentration with incipient cellular arrangement, while the second and third nephrostomes are only indicated as unorganized, tiny cell concentrations. During further development a gradual regression of the posterior portion of the glomus occurs, so that it finally becomes shorter than the pronephros. At stage 55, the first signs of degeneration appear also in the mesonephros, at the median side of its middle portion, where some mesonephric tubules and glomeruli have degenerated at stage 56. At stage 55 the wall of the posterior half of the Wolffian duct begins to alter.
