COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 201 



The lymphatic system contains a colorless fluid, the lymph, whose composition is similar 

 to the plasma of the blood. The lymph contains white blood corpuscles but red ones are 

 absent. The lymph fills aU of the spaces of the body and bathes all of the cells, and aU exchange 

 between the tissues and the blood must occur by way of the lymph. 



Owing to the delicate nature of the lymph vessels and the general diffuse character of the 

 lymphatic system, very little of this system can be made out in an ordinary dissection. Our 

 study of the circulatory system will therefore be confined almost entirely to the blood-vascular 

 system. 



2. The origin of the blood and of the blood vessels. The blood vessels and the blood 

 arise from the mesoderm, from mesenchyme cells. In the mesenchyme little patches of cells 

 form; the central cells of these patches become modified into blood corpuscles; the peripheral 



FIG. 51. Drawings of cross-sections through four successive stages of development of the chick 

 embryo, between 25 and 29 hours of incubation, to show the formation of the heart. A, early stage 

 showing the open intestine at c, the vitelline veins d in the splanchnic mesoderm/, and the thickening I in 

 the splanchnic mesoderm covering the veins. B, next stage in which the splanchnopleure has fused 

 together at m, closing the intestine c and the bringing the two vitelline veins d closer together. C, later, 

 showing the two vitelline veins d in contact. D, completion of the heart by the fusion of the vitelline 

 veins; the thickened mesoderm / becomes the muscular wall of the heart, a, neural tube; b, notochord; 

 c, anterior part of the digestive tract; d, vitelline vein; e, ectoderm; /, somatic and splanchnic layers of 

 the mesoderm; g, entoderm; h, dorsal mesentery of the heart or dorsal mesocardium; i, ventral mesen- 

 tery of the heart, or ventral mesocardium (it is disappearing in Z>) ; j, lining of the heart formed by the 

 union of the vitelline veins; k, pericardial cavity; /, muscle layer of the heart formed by the thickening 

 of the splanchnic mesoderm; m, point of fusion of the splanchnopleure of the two sides. (After Patten's 

 Early Embryology of the Chick, copyright by P. Blakiston's Son and Company.) 



cells arrange themselves to form tubes, the blood vessels. The blood vessels which arise in 

 the somatic mesoderm are somatic vessels; those in the splanchnic mesoderm are visceral or 

 splanchnic vessels. 



3. The origin of the heart. The heart arises in the ventral mesentery in the anterior 

 part of the embryo. In cases where the embryo is closed below from the beginning, a tubular 

 cavity appears in the ventral mesentery and in the walls of this cavity the heart tissue differ- 

 entiates. In the majority of vertebrates, with meroblastic development, the embryo is at 

 first open below on the yolk sac (see Fig. 8C and D, p. 40). In the splanchnic mesoderm of the 



