. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine. Physiology, Comparative. 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
. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine. Physiology, Comparative. 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.âDiagrams 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; â p. t., pro- nephric tubule; 50, somatic, and
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