MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 15 



was also reported by Solucha (1908). May the granular cells described by Boring 

 and Pearl as "interstitial cells" prove to be the normal constituents of the connective 

 tissue in the bird? Embryos with ubiquitous granuloblastic transformation of the 

 mesenchyme after splenic grafts seem to be particularly favorable for answering 

 this question. It has already been reported that after successful grafts of adult 

 splenic tissue on the allantois of a chick embryo 7 to 8 clays old the mesenchyme of 

 the host is subject to a remarkable proliferation with subsequent isolation and a 

 rapid hypertrophy of its cells, which further undergo a granuloblastic transforma- 

 tion. In spleen, thymus, allantois, and muscles great numbers of lymphoid 

 hemoblasts, granuloblasts, and granulocytes are formed. The granuloblastic line 

 of differentiation is imposed upon organs which, under normal conditions, manifest 

 a hemopoietic activity of a different kind; so, for example, the thymus, which nor- 

 mally is the center of production of small lymphocytes, now becomes a focus of 

 intensive granuloblastic activity. If the interstitial cells of Boring and Pearl are 

 merely the normal constituents of the connective tissue of the chick, closely related 

 to the family of blood-cells, we certainly would expect to find after grafts an enor- 

 mous increase of granuloblastic activity in the stroma of the sex-glands similar to 

 that found in other regions of the organism. It would, then, be an easy task to com- 

 pare this undoubtedly hemopoietic tissue with the cells described by Boring and 

 Pearl as interstitial. This task was greatly facilitated by the courtesy of Dr. Alice 

 Boring, who kindly made, at my request, a demonstration of her preparations and took 

 an active part in comparing the granular cells present in normal ovaries and testes 

 with the granuloblastic tissue developing in huge masses after splenic grafts. 



Figures 4, 5, 12, and 13 illustrate the striking changes in the stroma of the ovary 

 and testes after splenic grafts. If compared with the stroma of the sex-glands in a 

 normal chick embryo, in which only scattered amoeboid cells with acidophilic granules 

 can be found at this stage, the stroma in the sex-glands under investigation appears 

 to be remarkably dense and heavily infiltrated with amoeboid granular cells. Thick 

 walls of granuloblastic tissue surround the intima of vessels of any considerable 

 caliber. If it were not for the well-developed outer layer of the germinal epithelium 

 the structure of the ovary would hardly be recognizable, and I was obliged to con- 

 sult several well-known zoologists in order to definitely determine the nature of the 

 organ in question. 



Figure 5 represents a small part of the ovarian tissue adjacent to the germinal 

 epithelium. It is well vascularized, numerous capillaries being discernible; scanty 

 mesenchymal branched cells seem to hold them together. Two kinds of free cells, 

 different in structure and origin, are found in its meshes. A part of them are 

 unmistakably germ-cells; of large size and sometimes containing two vesicular 

 nuclei, their cytoplasm shows a peculiar opaque appearance, and with eosin and 

 azur II it acquires a purplish tint. But for the presence of distinct yolk-granules, 

 they may readily be identified with those "endodermic wandering cells" which in 

 1908 I described in chick embryos of 3 to 12 segments as situated in the anterior 

 part of the blastoderm between the ectoderm and the endoderm. At that time I 

 stated only that, though I found them occasionally within the vessels, they had 



