112 



COMPARATIVE PHYSIOLOOY. 



venae cavse superiores ; but in many mammals, among which is 

 man, the left vena cava superior disappears during foetal life. 



For the present we may simply say that the histories of the 

 development of the heart, the blood-vessels, and the blood itself 

 are closely related to each other, and to the nature and changes 

 of the various methods in which oxygen is supplied to the blood 

 and tissues, or, in other words, to the development of the respir- 

 atory system. 



THE DEVEIiOFMENT OF THE UROGENITAI. SYSTEM. 



Without knowing the history of the organs, the anatomical 

 relations of parts with uses so unlike as reproduction on the one 

 hand and excretion on the other, can not be comprehended; nor, 

 as will be shortly made clear, the fact that the same part may 

 serve at one time to remove waste matters (urine) and at an- 

 other the generative elements. 



The vertebrate excretory system may be divided into three 

 parts, which result from the differentiation of the primitive kid- 

 ney which has been effected during the slow and gradual evo- 

 lution of vertebrate forms : 



1. The head-kidney (pronephros). 



2. The Wolf&an body (mesonephros). 



3. The kidney proper, or metanephros. 



But in this instance, as in others to some of which allusion 

 has already been made, these three parts are not functional at 

 the same time. The pronephros arises from the anterior part 

 of the segmental duct, pronephric duct, duct of primitive kid- 

 ney, and archinephric duct, and in the fowl is apparent on the 

 third day ; but the pronephros is best developed in the ichthy- 



Fi«. 119.— Diagrams illustrating development of pronephros in the fowl (Haddon). 

 «o, aorta; o. c, body-cavity; ep, epiblast with its epitrichial (flattened) layer; hy, 

 hypoblast; m. s, mesoblastic somite; n. c, neural canal; nch, notochord; ■p. t., pro- 

 nephric tubule; 50, somatic, and s^, splanchnic mesoblast. 



