MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 13 



muscles must certainly be of importance in bringing about the various degrees of 

 infiltration of the muscles by hemopoietic tissue, since they all become starting 

 points of intensive proliferation. 



The future fate of the individual hemoblasts now situated within the muscle 

 can be easily predicted. Their localization outside the vessels excludes for them the 

 possibility of an erythropoietic differentiation. If in near proximity of vessels, 

 hemoblasts usually transform readily into granuloblasts. There is, indeed, an 

 intensive granuloblastic transformation of hemoblasts around the vessels traversing 

 the muscles. Heavy sheaths of granuloblastic tissue are gradually developing 

 around them. Granuloblasts (myelocytes), together with transitional stages of 

 their gradual transformation into granular leucocytes, are also found in great 

 numbers between the muscles and muscle fibers. Where, however, the connective 

 tissue is dense and poorly vascularized, the isolated amoeboid cells but seldom 

 manifest a tendency to elaborate acidophilic granules in their cytoplasm; and, if so, 

 they usually do not transform into granular leucocytes with rod-shaped granulation, 

 but retain their spherical or oval nuclei and develop minute acidophilic granules. 



Figure 3 is a drawing of a small part of a muscle in the neck of an embryo 

 fixed 5 days after a successful graft of adult splenic tissue on its allantois. Figure 

 9 represents a similar region in the photograph. In spite of the advanced stage of 

 granuloblastic transformation which a great number of hemoblasts have already 

 reached, the process of gradual development of a mesenchymal cell into its final 

 product, the granular leucocyte, can still be easily followed. This process affects 

 the hemoblasts in all the regions in which the structure of the tissue is not too dense 

 and which are supplied by a rich vascular net. Otherwise, the isolated amoeboid 

 cells appear in the form of histiotopic cells and retain this structure; quite remark- 

 able may appear their amoeboid activity, illustrated in figure 2. The larger accumu- 

 lations of hemoblasts found in the muscles also undergo a granuloblastic differ- 

 entiation. Granular leucocytes form in later stages the chief and sometimes the 

 exclusive constituents of these cell clusters. The granular leucocytes, not finding 

 an outlet into the circulation, undergo disintegration, and like in the spleen small 

 necrotic foci then ensue in the midst of the muscular tissue. 



One more region in the organism is found to contain mesenchyme which, at 

 least in an embryo of the seventh to eighth day of incubation, is equivalent to the 

 mesenchyme of hemopoietic centers and possesses a latent potency not revealed 

 under normal conditions. The mesenchyme here, as elsewhere, may be conceived 

 as disseminated hemopoietic buds, which, when awakened by a proper stimulus, 

 display a great proliferative and differentiating ability. How long these dormant 

 buds retain in the muscles their original metabolism and therefore their potentiali- 

 ties, and whether the latter may not be gradually lost as a result of a lasting sym- 

 biosis of the mesenchyme with the muscular tissue, can not be decided at present. 



MESENCHYME OF THE SEX-GLANDS. 

 The stroma of the sex-glands in different classes of animals has been made the 

 subject of special study by numerous investigators. In addition to the common con- 



