THE DEVELOPMENT OP THE EMBEYO ITSELF. 113 



opsida (flshes and amphibians). A vascular process from the 

 peritoneum (glomerulus) projects into a dilated section of the 

 body cavity, which is in part separated from the rest of this 

 cavity (fioalom). This process, together with the segmental duct, 

 now coiled, and certain short tubes developed from the original 

 duct, make up the pronephros. The segmental duct opens at 

 length into the cloaca. 



The meaonephros (Wolffian body), though largely developed 

 in all vertebrates during foetal life, is not a persistent excretory 

 organ of adult life. 



In the fowl recent investigation has shown that the Wolffian 

 (segmental) tubes originate from outgrowths of the Wolffian 



i i 



Piu. ISO. 



Fig. 181. 



Fio. 120. — Rndimentary primitive kidney of embryonic dog. The posterior portion of 

 the body of the embryo is seen from the ventral side, covered by the intestinal 

 layer of the yelk-sac, which has been torn away, and thrown back in front in 

 order to show the primitive kidney ducts with the primitive kidney tubes (a). 6, 

 primitive vertebrae; c, dorsal medulla; d, passage into the pelvic intestinal cavity. 

 (Haeckel, after Bischoff.) 



Fio. 121.— Primitive kidney of a human embryo, u, the urine-tubes of the primitive 

 kidney; w. Wolffian duct; w\ upper end of the latter (Morgagni's hydatid); m, 

 MfiUerian duct; m'. upper end of the latter (Fallopian hydatid); g^ hermaphrodite 

 gland. (After iSobelt.) 



duct and also from an intermediate cell-mass, from which latter 

 the Malpighian bodies take rise. The tubes, at first not con- 



8 



