The Embryogenesis of Lymphocytic Tissue 23 



(not thoroughly investigated in some of the lower forms) , and three types of 

 granulocytes. However, purely lymphocyte-forming organs (lymphatic nodes 

 and nodules) are found only in the mammals and in some water birds in 

 which they are limited to small, paired organs in the cervical and lumbar 

 regions. In adult birds lymphatic tissue appears as the white pulp of the 

 spleen and as nodules in parts of the connective tissue, in the lamina propria 

 of the gastrointestinal tract, in the bone marrow, and often in the liver. 



Below the mammals and birds there is no exclusively lymphatic tissue, that 

 is, there is no separation of lymphatic and myeloid tissues. The lymphocytes 

 develop intermixed with developing granulocytes in a variety of situations, 

 hence the name lympiio myeloid tissue. A striking example of this is in the 

 subcapsular tissue of the liver of urodeles; the same process occurs in the liver 

 of human embryos. 



In general, in the lower vertebrates, erythropoiesis occurs in organs other 

 than those in which the lymphomyeloid tissue is found. In these forms, just 

 as in reptiles and birds, erythropoiesis is usually intravascular. 



It is noteworthy that, within a given vertebrate species, the large and small 

 stem cells are morphologically indistinguishable, although there may be some 

 characteristic cytologic differences between species. 



Just as the development of purely lymphocyte-forming tissues and organs 

 occurs late in phylogeny, so, too, does it appear late in mammalian embry- 

 onic development. Nevertheless, it must be noted that (1) lymphocytes are 

 present in the blood of the lower vertebrates and (2) lymphocytes, even small 

 ones, develop very early in mammalian embryos. 



EMBRYOGENESIS OF MAMMALIAN BLOOD CELLS 



In the mammalian embryo (including that of man) blood cells develop in 

 and from the mesenchyme in a succession of sites, appearing first in the yolk 

 sac and the head and later in other parts of the body. Hematopoiesis next 

 appears in the liver, then in the bone marrow, and finally in lymph nodes and 

 spleen. In some species the thymus may be very important, as Dr. Matthew 

 Block tells me for his developing opossums. 



In all these situations, the hematopoietic process is much the same except 

 that the primitive erythrocytes are formed only in the body mesenchyme and 

 yolk sac (Slide 1) . With this exception, all the various types of blood cells 

 form in each of the situations, but the proportions of the different cell types 

 vary from site to site. There may be minor variations between species. Thus, 

 definitive erythrocytes are formed in great numbers in the yolk sac of the cat, 

 guinea pig, and rabbit but only in small numbers in that of man and the rat 

 (Slides 2-4) . In all mammals definitive erythrocytes are formed extensively in 

 the liver (Slide 5) and bone marrow (Slides 6 and 7). In most mammals 

 megakaryocytes and granulocytes are common in yolk sac and liver (Slide 5) 



