MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 25 



Very little is known about the mesenchymal constituents of the suprarenal. Kose, 

 however, mentions that elements of lymphatic tissue and sympathoblasts are 

 extremely similar. It is no longer difficult to draw a distinction between these 

 cells 6 to 7 days after grafting, all the elements of the lymphatic tissue having 

 undergone a granuloblast^ differentiation. Figure 6 shows a small part of the 

 adrenal tissue 7 days after the grafting of adult splenic tissue on the allantois of the 

 embryo. In addition to the erythrocytes, the capillaries at this stage contain a 

 great number of lymphoid hemoblasts. Granuloblasts are found outside the vessels, 

 among the groups of chromaffin cells, between the capillaries and the epithelial 

 strands, and even in the midst of these strands. The different kinds of cells are easily 

 identified by their structural characters. The sympathetic ganglion cells have 

 a structure similar to such cells elsewhere and usually are grouped together in 

 large numbers, though also found scattered between the chromaffin cells, as shown 

 in figure 6. The granuloblasts form irregular accumulations, sometimes large and 

 separating the tissue proper of the adrenal; usually, however, they are found scat- 

 tered, singly and in small groups. Occasionally lymphoid hemoblasts are found 

 and easily identified, their amoeboid nature offering a good criterion for their identi- 

 fication. It is difficult to determine the origin of the granuloblasts in the adrenal. 

 They might have been derived from the local mesenchymal cells or from the hemo- 

 blasts brought in by the circulation. Groups of granuloblasts (fig. 15, grbl.) situated 

 in the center of the epithelial strands are certainly cells which have come from other 

 regions. The complexity and density of the tissue in the adrenal primordium 

 obscures also the study of the early stages after grafting, during which the origin 

 of the granuloblasts tissue in other organs is usually easily determined. However, 

 the data obtained by study of the myeloid metaplasia of the mesenchyme in other 

 regions of the embryo make it probable that here, also, at least some of the granulo- 

 blasts are derived locally from the normal mesenchymal constituents of the organ. 



Circulating blood. — A few words will suffice to indicate the changes which the 

 circulating blood is undergoing during the myeloid metaplasia of the mesenchyme. 

 It has already been pointed out that in the early stages of the developing granu- 

 loblast! c transformation of the mesenchyme the blood in the vessels does not differ 

 from that encountered under normal embryonic conditions. As the hemoblasts 

 and granuloblasts develop in greater quantities, they accumulate in the stroma of 

 all the organs. As amoeboid cells they easily find access into the capillaries and 

 in later stages the blood in the vessels shows characteristic changes in the relative 

 numbers of its cells. The blood is undergoing changes which have been described 

 in leucaemias. Figure 17 illustrates the contents of the capillary net in the ad- 

 ventitia of a large vessel. A great number of hemoblasts and younger forms of 

 erythroblasts distend the capillary net. In still later stages granuloblasts and 

 granular leucocytes become especially numerous in the capillaries and veins and 

 the circulating blood offers then a picture characteristic of myeloid leucaemia. 



Loose mesenchyme of the body. — It is already known that the chick does not 

 possess lymph-glands. The lymphatic tissue is, however, present in the organism 

 in rather large amount. Instead of being well localized in the form of circum- 



