4^0 THE EVOLUTION OF MAN. 



appears as a body of considerable size. The posterior end 

 of the primitive kidney duct opens into the lower extremity 

 of the last section of the rectum, so that this organ becomes 

 a cloaca. But this opening of the primitive kidney duct 

 into the intestinal canal must be regarded, phylogenetically, 

 as a secondary condition. Originally, as is indicated clearly 

 in the Cyclostoma, they issued through the external abdo- 

 minal skin, quite independently of the intestinal canal, thus 

 proving their early phylogenetic origin from the horn-plate, 

 as outer skin glands. 



While in the Myxinoides the primitive kidneys per- 

 manently retain this simple form, as they do partially in 

 Primitive Fishes (Selachii), in all other Craniota it appears 

 only temporally in the embryo, as the ontogenetic repro- 

 duction of the primordial phylogenetic condition. In these 

 Skulled Animals the primitive kidney, by vigorous growth, 

 increases in length, and by the increase in number and the 

 coiling of the urinary tubes, very soon assumes the form of 

 a large compact gland, of oblong, oval, or spindle-shaped 

 form, which extends longitudinally through the greater 

 part of the body-cavity (coeloma) of the embryo (Figs. 123, m, 

 124, m, vol. i. p. 370). In this case, it lies near the middle line, 

 directly under the primitive vertebral column, and extends 

 from the region of the heart to the cloaca. The right and 

 left primitive kidneys lie parallel and close together, being 

 separated only by the mesentery, that narrow, thin lamella 

 which connects the central intestine with the lower surface 

 of the primitive vertebral column. The excretory duct of 

 each primitive kidney, the protureter, traverses the lower 

 and outer side of the gland in a posterior direction, and 

 opens into the cloaca, close to the root of the allantois ; at 



