726 THE CIRCULATORY SYSTEM 



2. Major Subdivisions of the Circulatory System 

 The circulatory system is composed of two major subdivisions: 



( 1 ) the arteriovenous system, composed of the heart, arteries, and veins 

 together with the blood vessels and capillaries of smaller dimensions 

 intervening between the arteries and veins, and 



(2) the lymphatic system, made up of lymph sacs, and lymphatic vessels 

 together with specialized organs such as the spleen, tonsils, thymus 

 gland, and lymph nodes. In larval and adult amphibia pulsating lymph 

 hearts are a part of the lymphatic system. Lymph hearts are present 

 also in the tail region of the chick embryo. 



The lymphatic vessels parallel the vessels of the arteriovenous system, and 

 one of their main functions appears to be to drain fluid from the small spaces 

 within tissues as well as larger spaces, such as the various divisions of the 

 coelomic cavity. 



The blood within the arteriovenous system is composed of a fluid substance 

 or plasma together with red blood corpuscles or erythrocytes, white blood 

 cells of various types, and blood platelets. The latter are small protoplasmic 

 bodies which may represent cytoplasmic fragments of the giant, bone-marrow 

 cells or megakaryocytes. The blood within the lymphatic system is composed 

 of a vehicle the lymph fluid, similar to the plasma of the arteriovenous blood 

 system, together with various white blood corpuscles. 



B. Development of the Basic Features of the Arteriovenous System 



1. The Basic Plan of the Arteriovenous System 

 The primitive circulatory system is constructed of three main parts: 



( 1 ) two sets of simple capillary tubes, bilaterally developed on either side 

 of the median line (fig. 332), 



(2) a local modification of these tubes which forms the rudimentary heart, 

 and 



(3) blood cells and fluid contained within the tubes. 



2. Development of the Primitive Heart and Blood Vessels 

 Associated with the Primitive Gut 



The primitive vascular tubes or capillaries form below the anterior region 

 of the developing metenteron or gut tube in relation to the yolk sac or yolk- 

 containing segment of the gut. Two sets of identical tubes begin to form, one 

 set on either side of the median plane of the embryo (fig. 332A and B). 

 Simultaneous with the formation of these primitive, subintestinal blood capil- 

 laries, the splanchnic layers of the two hypomeric portions of the mesodermal 

 tubes grow mesiad to cup around the blood capillaries in the area just posterior 



