The Embryogenesis of Lymphocytic Tissue 25 



in which case they or their progeny are called lymphocytes, or they will 

 develop into erythroblasts, some myelocytes and a few megakaryocytes. Later 

 the typical division of the organ into red and white pulp develops. In man 

 the formation of the myeloid elements ceases under normal conditions but 

 appears again in extramedullar) myelopoiesis. 



I have not discussed explicitly the nature of the primitive free stem cells 

 of the several hemocyte-forming areas and their relation to the question of 

 what is a lymphocyte. I know of no morphologic criteria by which these free 

 mesenchymal cells can be separated into various cell lineages until some of 

 them develop specific granules and become granulocytes, others elaborate 

 hemoglobin and become erythrocytes, and until still others, for example, 

 become plasma cells as shown by the specific fluorescent antibody technique. 

 I, for one, find it impossible to discriminate several types of free stem cells 

 when I study myelocytes developing along with lymphocytes (1) in a lymph 

 node primordium, (2) in the more obvious lymphomyeloid tissue of the 

 other embryonic mammalian organs as yolk sac, liver, or bone marrow, (3) 

 in lymphomyeloid tissues of the lower vertebrates, (4) in the bone marrow 

 with prominent lymphatic nodules in birds, or (5) in extramedullar) myelo- 

 poiesis in postnatal mammals when there is a reversion to the mixed lympho- 

 myeloid tissue. 



It seems to me that the sum of the comparative, embryologic, and experi- 

 mental cytologic evidence clearly points to the conclusion that in all blood 

 cell-forming situations the mesenchyme turns into free stem cells which give 

 rise to all the types of blood cells in all the primordia and that with the 

 progressive development of the embryo there is an apparent gradual de- 

 velopment, in some mammals, of a functional separation in postnatal life of 

 myeloid and lymphatic tissues. But this separation is not always permanent, 

 for in certain diseases in man and as a result of certain experiments, both 

 of these tissues may revert to the mixed lymphomyeloid or lymphomyelo- 

 er\ thropoietic tissue of the embryo. 



DESCRIPTIONS OF ILLUSTRATIONS 



Fourteen colored lantern plates illustrated some of the stages of the embryo- 

 genesis of mammalian blood cells in the original presentation of this chapter. 

 Seven of the slides were copied from published colored plates in articles by A. A. 

 Maximow, 6 9 W. Bloom, 1 - and W. Bloom and G. W. Bartelmez. 3 The othei 

 seven slides were color photomicrographs made expressly for the presentation. It 

 has not been possible to reproduce them in color here. It is better to relet spec ili- 

 cally to the original publications of the drawings rather than to reproduce them 

 here in black and white. The color photomicrographs would be useless in black 

 and white because the myelocytes developing in dense masses of lymphoid cells 

 would not show. 



