MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 27 



The bulk of the loose mesenchyme which occupies the axis of a growing feather is 

 seen to be transformed into granuloblastic tissue in figure 15. The cells of the young- 

 fat tissue (fig. 7) maintain their structure, but if mesenchymal cells remain un- 

 changed between them, they are transformed into hemoblasts and granuloblasts, as 

 shown in figure 7. 



Wherever accumulations of granuloblasts attain considerable size and the 

 final products of their further differentiation (the granular leucocytes) do not find 

 an outlet into the circulation, they disintegrate. If in small numbers, they are 

 ingested by the histiotopic wandering cells. If the centers of granuloblastic tissue 

 attain, however, a considerable size, and granular leucocytes are massed together, 

 the whole focus may become necrotic, and it is not a rare occurrence to find such 

 necrotic centers anywhere in the mesenchyme. They represent as many centers of 

 inclusions of non-split protein, which incite in the embryo a digestive activity in 

 whatever tissue is capable of manifesting it. 



CONCLUSIONS. 



The study of the various organs with granuloblastic transformation of the 

 mesenchyme leads naturally to the conclusion that the loose mesenchyme in an 

 embryo of 7 to 9 days' incubation is polyvalent and equipotential. The mesen- 

 chyme under normal conditions may manifest, in definite regions of the organism 

 and at definite stages, a hemopoietic potency, or may remain inert for an indefinite 

 time. It may, however, be so stimulated as to proliferate with the production of 

 numerous free cells and also to hypertrophy with a subsequent differentiation. 



The embryonal mesenchyme as late as 7 to 8 days of incubation in a chick 

 embryo is therefore capable of doing more than it does in typical development. Its 

 great assimilative and proliferative power, conferring upon its cells a potential 

 immortality, has been already established in tissue-cultures by Burrows and Carrel 

 (1911, 1912, 1913) for the embryo, and by Maximov for the adult (1916). The 

 culture method does not, however, offer to the connective-tissue cells conditions 

 adequate for their further so-called progressive differentiation. These cells outside 

 of the organism are evidently capable of retaining their metabolism in the culture 

 medium, for they do not show there any sign of qualitative changes. Their meta- 

 bolism in the cultures may evidently be very active, because they are capable of 

 not only maintaining themselves, but of intensely proliferating. So far, however, 

 it has not been determined by the cultural method whether a connective-tissue cell, 

 a derivative of the embryonal mesenchyme, may outside of the organism undergo 

 further differentiation. 



A preliminary step seems to be indispensable in any of the lines of hemopoietic 

 differentiation of the mesenchyme; i. e., a separation of the mesenchymal cellular 

 syncytium into single cells. The factors effecting this separation in the organism 

 have not been determined. The consistency of the medium was pointed out by 

 Uhlenhuth (1915) as a factor determining the shape of cells, a liquid medium round- 

 ing them up. The same factor is chiefly active in Rous's method of isolating indi- 

 vidual cells growing in the culture by tryptic digestion of the clot (1916). After 

 such treatment, growing connective-tissue cells, even though forming in the culture 



