566 INTRODUCTION TO AMNIOTA [CH. XXII 



In the kidney the mesonephros or sexual part is completely 

 separated from the metanephros, or non-sexual part, and the duct 

 of the latter, the ureter, is formed in a different manner from that 

 in which the metanephric duct or ducts of Urodela and Elasmo- 

 branchii are constituted. For whereas the metanephric ducts of 

 these two groups are formed by a longitudinal splitting of the 

 archinephric duct which separates the hinder kidney tubules from 

 the sperm-carrying portion of the duct, the ureter of Amniota 

 is formed as a flask-shaped evagination or outpouching of the 

 archinephric duct primitively at right angles to its course. The 

 body of the flask forms a cavity of the kidney termed the 

 pelvis of the kidney, round which the metanephric tubules 

 are grouped and into which they open; the neck of the flask 

 which becomes continually lengthened forms the ureter; and the 

 lower short part of the archinephric duct, common to it and to 

 the sperm duct, becomes absorbed into the cloaca so that both 

 ducts open independently of each other (Fig. 216). When we 

 review this list of differences we see that whereas some of them 

 could be regarded as evolutions of and improvements on the 

 Amphibian type of structure, yet there are others, such as the con- 

 stitution of the skull and the vertebral column, in which evolution 

 seems to have gone on independently in Amphibia and Amniota, 

 and to find a common starting-point for the two lines, we must 

 go back to the very oldest land animals. Amongst the extinct 

 creatures termed Stegocephala and reckoned as Amphibia, the 

 earliest had no centra and the arch-pieces remained separate, and 

 in some of these there is some evidence that the basi-occipital 

 element in the skull was ossified and that the ventral intercalary 

 was becoming large and important, and amongst these we must 

 look for the ancestors of Amniota and the point of cleavage of the 

 stock of land animals into two divergent stems. 



