MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 



11 



in the interstices between the groups of muscle-cells should prove to be identical in 

 both cases, it would seem that muscular cells at an early stage of their development 

 can, by a reversible process, reassume the structure and potencies of the mesen- 

 chyme. The cells of this mesenchyme are found soon to increase their cytoplasm, 

 which at the same time begins to appear more basophilic. The mesenchymal 

 syncytium occupying the interstices and that closely adjacent to the muscle-fibers 

 are not two separate structures and are seen to be directly connected by cytoplasmic 

 processes. The intense proliferation observed in the mesenchyme leads soon to 

 the formation here of a dense cellular tissue. The cells are gradually drawn 

 together by the increase in their number as well as in size. In some places the 

 increasing condensation of the mesenchyme transforms it finally into plasmodial 

 masses, in which individual cellular units are no longer distinguishable. Moreover, 

 a marked hypertrophy of numerous cells imposes at this time a polymorphic char- 

 acter upon the formerly uniform syncytium. The aspect of a muscle of an embryo 

 under experiment at this stage differs already to a great extent from the structure 

 of a normal muscle of a control embryo. 



This mesenchyme soon acquires an even more peculiar aspect, while numerous 

 cells of the syncytium withdraw their processes from the common network, become 

 free, and appear under the form of amoeboid cells. This process, though more pro- 

 nounced in the mesenchyme situated in the spaces between the muscle-fibers, 

 affects equally those mesenchymal cells which are closely adjacent to the muscular 

 fibers. It is difficult to determine whether or not single cells between parts of 

 muscles in a normal embryo may not be detached from the syncytium and remain 

 here as dormant wandering cells. The process of separation of amoeboid cells from 

 the mesenchymal syncj'tium in different parts of the loose mesenchyme and in 

 different blood-forming organs is now widely accepted. The rather unique locali- 

 zation of this process in the muscles, even between its individual fibers, and its 

 great intensity revealed here, distinguish this process from that observed elsewhere 

 under normal conditions. The cells detached from the mesenchymal syncytium 

 in their subsequent development clearly show that in the stage of free cells they are 

 capable of a more vigorous assimilation and of a higher synthetic power and that, 

 at the same time, they more readily respond to external influences by fundamental 

 structural changes. The separated mesenchymal cells hypertrophy and proliferate 

 intensely. 



Figure 2 gives an idea of the intensity which the process of proliferation and 

 isolation of mesenchymal cells may attain in the muscles of some regions. This 

 figure represents a small part of a cleft between the muscle-bundles in a tangential 

 aspect . The group of muscular fibers are surrounded here by a dense accumulation 

 of amoeboid cells, considerable numbers of which seem to fuse into plasmodial 

 masses. Many among these cells have already acquired the structural characteris- 

 tics of hemoblasts; other amoeboid cells resemble more the wandering cells of the 

 connective tissue, which some years ago (1909) I described in the loose mesenchyme 

 of the bird and reptile, under the name of histiotopic wandering cells. In these 

 accumulations of free cells found between the muscle-fibers, the distinctive char- 



