brates these are spindle cells resembling the lymphocytes; 

 in mammals cell fragments or platelets serve this function. 



In the mammal, hemopoietic (blood-cell producing) tis- 

 sue is of two types, myeloid and lymphatic. The myeloid 

 tissue produces erythrocytes, granular leukocytes, and plate- 

 lets. Lymphatic tissue produces nongranular leukocytes. 

 Myeloid tissue is confined to the bone marrow in the mam- 

 mal; lymphoid tissue occurs as lymph nodes in the visceral 

 connective tissue or gut wall, hemal nodes, the tonsils, thy- 

 mus, and spleen. 



In the embryo, there is change in the area of origin of 

 blood cells. The first cells are those derived from the blood is- 

 lands of the yolk sac. Here clumps of mesenchyme cells dif- 

 ferentiate to form vesicles containing a fluid and suspended 

 cells. Some of these cells contain hemoglobin but are nu- 

 cleated, resembling early stages of the erythrocytes of the 

 adult (see 10-mm pig, serial sections). The differentiation of 

 the blood islets has been viewed in two ways, as described 

 above and as a syncytium which becomes cellular. 



The vesicles formed by the blood islands tend to join and 

 form channels. In this way the reticulum of the yolk sac 

 develops and enters the body as the vitelline veins from 

 which the heart and the first aortic arch form. 



The venous channels of the body appear to arise in situ 

 from clumps of mesenchyme cells which form into channels 

 for the return flow of fluid to the yolk sac or heart. At first 

 the arterial and venous channels are directly connected. 

 With the beating of the heart, the fluid is put under pres- 

 sure and given a direction of flow. This circulation move- 

 ment determines the development of outgoing channels as 

 well as return channels. Direct connection between arteries 

 and veins is discouraged by the outgrowth of their walls 

 into the tissues as a branching system of vessels. The vessels 

 are connected at all times through the tissue spaces until the 

 final connection through lined capillaries is achieved. 



As the embryo differentiates, blood cells are derived from 

 mesenchyme clumps along the blood channels throughout 

 the body. Many new cells arise by division of circulating cells. 

 As the embryo gets older new types of cells appear. With the 

 formation of the liver, spleen, and lymphoid tissue of the 

 throat, cell production shifts to these sites. In the mammal 

 these early areas, along with the lymph nodes, retain only 

 the capacity to produce nongranular leukocytes. The bone 

 marrow takes over the production of erythrocytes and gran- 

 ulocytes (third month and later in human). All of these 

 areas are active in the phagocytosis of damaged blood cells. 



In the reptile and bird, the pattern of blood-cell produc- 

 tion parallels somewhat that of the mammal, but the kidney 



is an important embryonic source. The adult bone marrow 

 produces all kinds of cells including the nongranular leuko- 

 cytes. Lymph nodes are absent although a lymphatic system 

 is present. In the amphibians the red cells are formed main- 

 ly in the kidney and destroyed in the liver and spleen (en- 

 dothelioreticular tissue). The bone marrow may be an im- 

 portant red-cell forming center of the adult, and it may be 

 only a seasonal source (males in breeding season). The 

 spleen is a source of lymphocytes, although there is little 

 white pulp in it. 



In the fishes there is a variety of hemopoietic tissues. 

 The first cells arise from the yolk sac, followed by cells 

 derived from the mesenchyme of the body vessels. An inter- 

 mediate cell mass, along the notochord in the kidney region, 

 produces blood cells. These areas are followed by the devel- 

 opment of the liver and spleen, but primarily the kidney. 

 Lymphoid tissue is scattered in the intestinal wall, in the 

 mesentery supporting the gut, and the thymus. 



In many teleosts the kidneys appear to be the most im- 

 portant blood-cell producing structures, and in some the 

 spleen becomes an organ functioning in the removal of blood 

 cells. In the sturgeons there is a large pericardial mass of 

 hemopoietic tissue, while in Amia and Lepisosteus the cover 

 of the fourth ventricle appears to be involved in this func- 

 tion. 



In the lamprey and myxinid, blood-cell production seems 

 to be limited to the intestinal wall and mesenteries. In the 

 lamprey the kidney of the adult and the typhlosole in the 

 gut of the larva are important centers. 



General observations 



Although blood-cell production is of interest to the em- 

 bryologists and the shifting of sites of blood-cell production 

 indicates an evolutionary history, this area of information 

 has not supplied any usable clews to the study of phylogeny. 



THE LYMPHATICS 



A part of the functional circulatory system is the lymphatic. 

 The intercellular spaces are drained by thin-walled channels 

 which eventually drain into the blood circulatory system. In 

 the mammal the vessels of this system are associated with 

 lymph and hemal nodes, and there is an extensive system of 

 vessels permeating the tissues. Similar systems are observed 

 in all vertebrates, but the difficulties of observation do not 

 encourage comparative study. 



378 



THE CIRCULATORY SYSTEM 



