26 MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 



scribed agglomerations of lymphatic tissue, the lymphatic elements are scattered 

 and diffusely infiltrate the loose connective tissue. If grouped in larger numbers 

 the accumulations of lymphatic cells may attain a size just large enough to be 

 discernible by the naked eye. The patches of lymphatic tissue disseminated in the 

 loose connective tissue begin to appear in the later stages of embryonal life, but 

 attain their full development only after hatching. I have already had occasion, in 

 one of my previous papers (1916c), to briefly sketch their structure and origin and 

 also to point out the fact that the loose mesenchyme, which in the typical develop- 

 ment of chick differentiates into lymphatic cells, may after grafts follow another 

 line of differentiation — i. e., to transform into granuloblastic tissue. 



Wherever loose mesenchyme is present in the embryo numerous cells are seen 

 after grafts of adult splenic tissue to change their shape from stellate to spherical, 

 and to free themselves. Part of these cells hypertrophy, soon become basophilic, 

 and appear in the form of lymphoid hemoblasts. Others, though attaining a con- 

 siderable size, nevertheless remain slightly basophilic and are often seen to protrude 

 small and thin processes on their whole surface. These cells are the histiotopic or 

 resting wandering cells. Both types, the hemoblastic and the histiotopic wandering- 

 cells, in the early stages after grafting, are represented in great numbers in the loose 

 mesenchyme. Swarms of amoeboid cells wander around and quite frequently 

 intermediate forms are seen, leaving no doubt as to their easy transition. As 

 previously stated, the hemoblastic type develops preferably around large, thin- 

 walled vessels with slow circulation. The histiotopic or resting wandering type, 

 on the contrary, is encountered more frequently in regions with scanty vessels. 



Both types of cells may be found scattered or grouped together in large or 

 small agglomerations. The histiotopic wandering cells manifest a tendency to flow 

 together and to form plasmodial masses. Individual cells may secondarily detach 

 themselves and acquire the structural features of the hemoblastic type. Seldom 

 do the cells of the histiotopic type manifest a tendency to granuloblastic differ- 

 entiation without previously having transformed into a lymphoid hemoblast. One 

 may find single wandering cells of this type containing a couple of acidophilic gran- 

 ules, but it is difficult to decide whether these granules have been elaborated by the 

 cell itself or whether they are remnants of ingested and digested cells. The histio- 

 topic wandering cells, wherever they are situated, display great phagocytic activity. 

 This activity in the particular case is directed chiefly against granular leucocytes 

 in the loose connective tissue which do not find an outlet into the circulation. 



Agglomerations of lymphatic hemoblasts are frequently observed in the loose 

 mesenchyme and they regularly undergo a granuloblastic transformation. Agglom- 

 erations of such cells develop chiefly in the neighborhood of vessels. Figure 8 

 shows a longitudinal section of a vessel, accompanying a small nerve. Both struc- 

 tures are surrounded by strands of hemoblasts. Hemoblasts are found even within 

 the small nerve. In figure 16 large accumulations of hemoblasts under the mucosa 

 of the pharynx are seen to have already transformed into granuloblasts and the 

 normally loose tissue is made heavy and dense by the presence of innumerable 

 amoeboid cells in various stages of granuloblastic transformation. The adventitia 

 of a vessel transformed into a heavy coat of granuloblastic tissue is seen in figure 1 1 . 



