68 



ANATOMY OF AMPHIOXUS. 



Fig. 32. Pronephric system of embryo of Ichthyophis, reconstructed from sec- 

 tions, and represented as having been spread out in one plane. (After SEMON.) 



a. Dorsal aorta, c. Portions of the ccelom into which the nephrostomes of the 

 pronephric tubules open. The inner portion of ccelom (next to aorta) is shut off 

 from the rest of the ccelom, and becomes associated with the vascular outgrowths 

 from the dorsal aorta (which produce the glomeruli) to form the Malpighian cap- 

 sules of the pronephros. The Malpighian tractus is continued backwards as 

 a metamorphosed and rudimentary cord of cells, nearly to the cloaca, and con- 

 stitutes the so-called Nebenniere or Interrenal body. This backward extension 

 of the Malpighian body of the pronephros probably indicates the former existence 

 of a much more extensive pronephric system, p. Convoluted pronephric tubules 

 lying above the peritoneum (shaded light), each provided with two nephrostomes, 

 inner and outer, and opening peripherally into d, the longitudinal pronephric duct 

 (Wolffian duct), which becomes the mesonephric duct after the degeneration of 

 the pronephric tubules and the formation of the mesonephric tubules have taken 

 place, m. Rudiments of the mesonephric tubules. 



N.B. The pronephric tubules are here characterised by the possession of 

 ccecal outgrowths. 



