Whitman the entire nephridial system (in the leech Clepsine) is formed by the differentiation of a continuous epiblastic band on each side.
Bergh (for Lumbricus and Criodrilus), whose figures show a derivation of the entire nephridium from mesoblast, and an absence of any connexion between successive nephridia by any continuous band, epiblastic or mesoblastic. A midway position is taken up by Wilson, who asserts the mesoblastic formation of the funnel, but also asserts the presence of a continuous band of epiblast from which certainly the terminal vesicle of the nephridium, and doubtfully the glandular part of the tube is derived.
The rhynchocoel is formed by a split which appears in the mesoblast surrounding the epiblastic pit which is the forerunner of the proboscis.
The excretory system is epiblastic in its origin.
24, in optical median section, showing the invaginated cells hy which form the arch-enteron, and the mesoblastic cells me which are budded off from the surface of the mass hy, and apply themselves to the inner surface of the epiblastic cell-layer cp. C, The same embryo focused so as to show the mesoblastic cells which immediately underlie the outer cell-layer.
The ligamentum denticulatum is thus lined on one side by the epiblastic atrial epithelium, and on the other by mesoblastic coelomic epithelium.
While still within the egg-membrane the epiblastic cells become flagellated, and the gastrula rotates within the membrane.