18 MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 



and the ovaries is certainly equivalent to that of other regions of the body already 

 examined, at least in respect to its granuloblasts potency. 



MESENCHYME OF THE KIDNEYS. 



The potentialities of the mesenchyme of two organs — the mesonephros and 

 metanephros — have to be considered under this heading. Both organs are present 

 at 13 to 1 5 days of incubation (5 to 7 days after grafting), at which time the changes 

 in the mesenchyme of the embryos under experiment were well advanced and the 

 embryos studied. Its well-established origin and history render the study of the 

 mesenchyme in both organs particularly interesting. 



At an early embryonic stage the primordium of the epithelial and mesenchymal 

 constituents of the kidneys are represented partly by the segment stalks and partly 

 by that portion of the mesoderm which corresponds to them and which, in the 

 chick, is transformed caudally from the twentieth or the twenty-first segment into 

 the nephrogenic cord. Its anterior portion, the mesonephrogenic cord, participates 

 in the formation of the mesonephros, and its shorter caudal portion separates and 

 forms the primordium of the metanephros. The connective tissue of the meso- 

 nephros is derived from the dorsal parts of the segment stalks or of the nephrogenic 

 cord, the cells of which lose their epithelial structure and form a loose mesenchymal 

 tissue. It is also the dorsal part of the metanephrogenic cord, known as the external 

 zone of the cord, which gives rise to the connective tissue of the metanephros 

 (Felix, 1906). The mesenchyme of both organs, therefore, originates from that 

 part of the mesoderm which is intimately connected with the somatopleura. 



The mesenchyme of the kidneys, abundant in the early stages of embryonic 

 development, becomes markedly scarce with the differentiation of new secreting 

 units of the kidneys and with their further convolution. In the fully developed 

 mesonephros, as well as in the adult metanephros, the mesenchyme is scarce and 

 usually described as consisting of isolated fibroblasts and fibrils which invest the 

 blood-vessels and the renal tubules. Under pathological conditions the stroma of the 

 adult kidney can be seen to become much more abundant, but neither in adult nor 

 in embryonic life was the stroma of the kidneys seen to assume a hemopoietic 

 activity. Development of bone and bone-marrow was described in the kidney of 

 the adult rabbit by Sacerdotti and Frattin (1902), and by Poscharissky (1905), the 

 differentiation of the bone being attributed to the activity of the connective tissue. 

 The development of the blood-cells has been, however, interpreted as a further differ- 

 entiation of the lymphocytes, brought into the organ by the blood-stream (Maximov, 

 1907 ) . In lower animals only does the mesenchyme of the mesonephros seem to be 

 regularly connected with hemopoietic function. The stroma of the mesonephros 

 in fishes becomes the seat of a permanent blood-forming activity, but neither in 

 birds nor in mammals, as already stated, does the mesenchyme of the kidneys nor- 

 mally exhibit any of the hemopoietic potencies. The inference, therefore, that 

 the hemopoietic potency exercised by the mesenchyme of the kidneys at a definite 

 period of its evolution is gradually lost in higher vertebrates would seem to be well 



