Fig. 388—Pig-embryo, three-fifths of an inch long, seen from the ventral side. a fore leg, z hind leg, b ventral wall, r sexual prominence, w nephroduct, n primitive kidneys, n1 their inner part. (From Oscar Schultze.)
Fig. 389—Human embryo of the fifth week, two-fifths of an inch long, seen from the ventral side (the anterior ventral wall, b, is removed, the body-cavity, c, opened). d gut (cut off), f frontal process, g cerebrum, m middle brain, e after brain, h heart, k first gill-cleft, l pulmonary sac, n primitive kidneys, r sexual region, p phallus (sexual prominences), s tail. (From Kollmann.)
As in the morphology of any other system of organs, so in the case of the urinary and sexual organs the Amphioxus is the real typical primitive Vertebrate; it affords the key to the mysteries of the structure of man and the higher Vertebrates. The kidneys of the Amphioxus—first discovered by Boveri in 1890—are typical “fore kidneys,” composed of a double row of short segmental canals (Fig. 217 x). The inner aperture of these pronephridia opens into the mesodermic body-cavity (the middle part of the cœloma, B); the external aperture into the ectodermic mantle or peribranchial cavity (C). Their position, their structure, and their relation to the branchial vessel make it clear that these segmental pronephridia correspond to the rudimentary fore kidneys of the Craniotes. The mantle-cavity into which they open seems to correspond to the prorenal duct of the latter.
Figs. 390, 391, 392—Primitive kidneys and rudimentary sexual organs. Figs. 390 and 391 of Amphibia (frog-larvæ); Fig. 390 earlier, 391 later stage. Fig. 392 of a mammal (ox-embryo). u primitive kidney, k sexual gland (rudiment of testicle and ovary). The primary nephroduct (ug in Fig. 390) divides (in Figs. 391 and 392) into the two secondary nephroducts—the Mullerian (m) and Wolffian (ug′) ducts, joined together behind in the genital cord (g). l ligament of the primitive kidneys. (From Gegenbaur.)