12 MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 



acters of both groups of cells intermingle so gradually as to make this distinction in 

 numerous cases no longer warranted. A constant relation between the characters 

 of the amoeboid cells (histiotopic or hemoblastic) and their topographic situation 

 can be usually discerned. In the proximity of vessels the amoeboid cell acquires 

 preferably the structure of the hemoblast ; in a region scantily supplied with vessels 

 isolated mesenchymal cells appear in the form of histiotopic wandering cells. In 

 regard to their developmental potencies these cells must be considered equipoten- 

 tial; first, because they so easily interchange; second, because they both manifest 

 identical structural changes, as will be seen later. 



Figure 8 illustrates the changes in the mesenchyme of the muscles in the chick 

 leg under low-power. The heavy strands which occupy the interstices between the 

 groups of muscular fibers can be identified under high power as accumulations of 

 hemoblasts. In some parts of the photograph the cells are so closely drawn together 

 that they give the impression of forming uninterrupted sheets, and only with the 

 aid of high power can the reciprocal boundaries and the highly amoeboid character 

 of the respective cells be recognized. 



Mesenchymal septa between large parts of individual muscles are equally 

 affected, and at later stages appear densely infiltrated by mesamoeboid cells. Even 

 a more intense proliferation and separation into free cells is manifested by the 

 mesenchymal tissue around the vessels traversing the muscle. Here whole sheaths 

 of considerable thickness develop around the vessels and follow their ramifications. 

 The study of the early stages of the myeloid metaplasia of the mesenchyme 

 is particularly instructive as undoubtedly illustrating the local origin of the 

 hemoblasts. At the time when an intensive development of hemoblasts takes 

 place around the vessels, the blood within the vascular walls consists chiefly of 

 erythrocytes in a more or less advanced stage of development and carries only single 

 specimens of undifferentiated stem-cells or hemoblasts. The study of such stages 

 leaves no doubt as to the local origin of the large accumulations of amoeboid cells 

 within the muscles. 



Changes similar to those described above extend rapidly to the mesenchyme 

 of the whole body, and as early as 4 or 5 days after a successful grafting the muscles 

 present a peculiar aspect. A closer comparative study of different muscle-groups 

 reveals that not all of them are affected to the same degree. Muscles can be found 

 in which healthy muscular fibers become scarce and seem lost in huge accumula- 

 tions of hemopoietic tissue. Sometimes direct atrophy and destruction of muscular 

 tissue is brought about by development of large, rather well-circumscribed centers 

 of hemopoietic tissue. Again, in other regions muscles do not differ much from 

 their usual appearance. A direct ingrowth of cells from the mesenchyme surround- 

 ing the more superficial group of muscles can, in some cases (for example in the 

 muscles of the neck), sufficiently explain the striking changes in them (figs. 3 and 9) ; 

 a closer observation, however, does not reveal a regular relation between localiza- 

 tion of the muscles and intensity of changes in them. The differences in the amount 

 of connective tissue and the number of septa and septula normally present in the 



