766 THE CIRCULATORY SYSTEM 



A characteristic feature of the lymphatic system is the development of 

 lymph nodes (lymph glands) along the lymphatic vessels. A lymph node is 

 a small, rounded structure with lymph vessels entering it at various points 

 (fig. 343E). From these lymph vessels, a flow of lymph oozes around a 

 meshwork of lymphoid cords, contained within the lymph node. After pass- 

 ing through the meandering lymph spaces within the node, the lymph emerges 

 from the opposite side of the lymph node into lymphatic channels. 



Lymph nodes appear to arise from lymph sacs which are invaded by in- 

 growing mesenchyme and connective tissue. Lymphoblasts become associated 

 with these connective-tissue ingrowths, and lymphocytes are differentiated in 

 large numbers. Eventually the developing lymph node forms two areas, an 

 outer cortex, containing dense masses of lymphocytes and an inner medulla, 

 containing a loose meshwork of lymph channels and sinuses. Connective tissue 

 forms a capsule around the lymph node from which partitions or trabeculae 

 grow inward to divide the cortex into secondary nodules. Beneath the capsule, 

 a peripheral lymph sinus is developed. Blood vessels enter the lymph node 

 at the hilus and pass along the trabeculae to the secondary nodules. The re- 

 turning blood vessels follow the same pathways. 



The spleen is a large lymph gland attached to the omental derivative of the 

 dorsal mesogastrium or peritoneal support of the stomach. It arises as a 

 concentration of mesenchyme along the left aspect of the early mesogastrium. 

 This mesenchymal mass eventually increases in size and projects from the 

 surface of the mesogastrium from which it later becomes suspended by a 

 constricted peritoneal support, the gastro-splenic ligament. 



The mesenchymal mass of the developing spleen is well supplied with 

 blood vessels, and a completely closed set of vascular channels is formed at 

 first. Later, however, sinus-like spaces appear which unite with the closed 

 vascular channels converting the closed system into one possessing open 

 sinuses. Lymphoid tissue forms and masses of splenic corpuscles develop 

 about the blood vessels. (Consult Maximow and Bloom, '42, for detailed 

 description of splenic structure.) 



F. Modifications of the Circulatory System in the Mammalian Fetus 



at Birth 



Consult Chap. 22. 



G. The Initiation of the Heart Beat 



The first parts of the heart to be developed are the anterior regions, namely, 

 the bulbus cordis and the ventricle. When the ventricular region is developed 

 in the chick, it starts to twitch. Later when the atrial portion is formed, it 

 commences to contract with a rhythm different from that of the ventricular 

 area, and its beat supersedes that of the ventricle. Still later when the sinus 

 venosus is established, it emerges with its own contraction rhythm, and this 



