INTRODUCTION 725 



c. Changes in the primitive converging veins of the heart in the chick 



1 ) Transformation of the vitelline and allantoic veins 



a) Vitelline veins 



b) Allantoic veins 



2) Formation of the inferior vena cava 



3) Development of the precaval veins 



d. The developing converging veins of the mammalian heart 



3. Development of the heart 



a. General morphology of the primitive heart 



b. The basic histological structure of the primitive embryonic heart 



c. Importance of the septum transversum to the early heart 



d. Activities of early-heart development common to all vertebrates 



e. Development of the heart in various vertebrates 



1 ) Shark, Squalus acanthias 



2) Frog, Rana pipiens 



3) Amniota 



a) Heart of the chick 



b) Mammalian heart 



( 1 ) Early features 



(2) Internal partitioning 



(3) Fate of the sinus venosus 



(4) The division of the bulbus cordis (truncus arteriosus and conus) 



f. Fate of embryonic heart segments in various vertebrates 



4. Modifications of the aortal arches 



5. Dorsal aortae (aorta) and branches 



E. Development of the Lymphatic System 



F. Modifications of the circulatory system in the mammalian fetus at birth 



G. The initiation of the heart beat 



A. Introduction 



1. Definition 



Living matter in its active state depends for its existence upon the beneficent 

 flow of fluid materials through its substance. This passage of materials con- 

 sists of two phases: 



( 1 ) the inflow of fluid, containing food materials and oxygen, and 



(2) the outflow of fluid, laden with waste products. 



In the vertebrate grqup as a whole, the inflow of materials to the body 

 substance occurs through the epithehal membranes of the digestive, integu- 

 mentary, and respiratory systems, while the outflow of materials is effected 

 through the epithelial membranes of the excretory, respiratory, and skin sur- 

 faces. The passage of materials through the substance of the body lying 

 between these two sets of epithelial membranes is made possible by (a) the 

 blood and (b) a system of blood-conveying tubes or vessels. These structures 

 form the circulatory system. 



