154 HISTOLOGY. 



Such marrow as has just been described is called red marrow. It 

 occurs in the bones of embryos and persists in the flat bones of the adult, 

 those of the skull, the bodies of the vertebrae, the ribs and sternum, 

 the epiphyses, and the heads of the humerus and femur. The shafts of the 

 long bones contain yellow marrow which resembles ordinary fat -tissue. 

 Between the fat cells an occasional plasma cell or myelocyte may occur. 

 Yellow marrow is formed from red by the development of true fat cells 

 and not by fatty degeneration of myelocytes. In disease it may resume 

 its, blood forming function and become red. In starvation it becomes 

 mucoid like other fat tissue. 



LYMPH NODULES AND LYMPH GLANDS. 



Lymph glands, haemolymph glands, and the spleen have a similar 

 origin in the embryo. They are at first small dense areas of mesenchyma 

 developing near blood and lymphatic vessels. The blood vessels extend 

 into these areas producing a notch on one side of the mass, known as the 

 hilus. Here in the adult the arteries enter and veins leave. After the 

 invasion of the blood vessels the dense tissue is transformed into reticular 

 tissue containing lymphocytes. The lymphocytes occur especially in that 

 part which surrounds the arteries. The veins tend to be at the periphery 

 of the compact lymphoid tissue surrounding the arteries and to be asso- 

 ciated with a portion of the reticulum which is comparatively free from 

 lymphocytes. Lymphatic vessels spread over the surface and into the 

 substance of the lymph glands, but they are absent from haemolymph 

 glands and from the spleen. 



Lymph glands (also called lymph nodes) in early stages of develop- 

 ment are shown in Fig. 175; the left half of which represents a younger 

 stage than the right. The left portion shows a mass of reticular tissue 

 and lymphocytes penetrated by an artery and a vein which join through 

 capillaries. It is surrounded by a network of lymphatic vessels some of 

 which are afferent and others, toward the hilus, are efferent. Such 

 structures occurring in the adult are called solitary nodules [follicles]. They 

 are abundant in the walls of the intestine and respiratory tubes. Each 

 is an area of lymphocyte production characterized by crowded nuclei 

 w r hich stain deeply with haematoxylin. Under low magnification the 

 nodule appears as a mass of dark granules (Fig. 244, p. 216) in the center 

 of which a lighter area is sometimes seen, the germinaiive center. Here 

 the cells are larger, resembling the large mononuclear leucocytes of the 

 blood, and are frequently found in mitosis. They are thought to give 

 rise to lymphocytes. The reticular tissue, which is concealed by the cells 



