106 ANIMAL PHYSIOLOGY. 
Later an incomplete septum forms similar divisions in the auri- 
cle; the aperture (foramen ovale) left by the imperfect growth 
of this wall persisting throughout feetal life. 
The Eustachian valve arises on the dorsal wall of the right 
auricle, between the vena cava inferior and the right and left 
ven cavee 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 DEVELOPMENT OF THE UROGENITAL 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 
kidney which has been effected during the slow and gradual 
evolution of vertebrate forms: 
1, The head-kidney ( pronephros). 
2. The Wolffian 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 ich- 
thyopsida (fishes 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 (celom). 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. 
