24 MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 



perineurium are found 5 to 7 days after grafting to be thickly infiltrated with, if 

 not entirely transformed into, mobile granular cells. A study of the early stages 

 in the development of this granuloblastic tissue leaves no doubt as to its local 

 origin. The heavy layers of granuloblastic tissue into which the adventitia and 

 the perineurium are transformed, as well as the discrete strands of the same tissue 

 in other organs, are the products of an intense proliferation of mesenchymal cells 

 detached from the local mesenchymal syncytium and undergoing further unavoid- 

 able differentiation. This is not an outgrowth of settlers brought into the different 

 organs by the blood-current. The isolation of the mesenchymal cells (primarily 

 in syncytial and even in plasmodial connections with each other), their prolifera- 

 tion, and their further granuloblastic transformation are processes which do not 

 necessarily require observations on living tissue under the microscope and are 

 easily determined in preparation of adequately fixed and stained material. A 

 further corroboration of the local origin of the granuloblastic tissue is given by a 

 study of the circulating blood in the early stages of the mesenchymal transforma- 

 tion. At the time when great numbers of amoeboid cells are liberated in the mesen- 

 chyme the circulating blood, both in the large and small vessels, is normal and con- 

 sists of more or less completely differentiated elements. Only in well-advanced 

 stages of the myeloid metaplasia of the mesenchyme, as will be seen later, does the 

 blood reflect the processes taking place in the mesenchyme. At this time the 

 blood in the embryos under experiment offers a picture very much like that found 

 in the leucaemia (fig. 17). 



Mesenchyme of the liver. — Figure 10 shows the changes found in the liver at 

 the end of the seventh day after grafting. The connective tissue in the bird liver 

 being very scanty, nevertheless constitutes a starting-point for the liberation of 

 amoeboid cells which subsequently proliferate and differentiate. Accumulations 

 of hemoblasts and granuloblasts are formed and surround every vessel in the form 

 of a heavy coat. Single rows of hemoblasts and granuloblasts are often found 

 between the capillaries and the liver cells. They are probably derived from mesen- 

 chymal cells analogous to those which in the adult mammal are known as Kupffer 

 cells. In an advanced stage of granuloblastic transformation of the mesenchyme 

 in the liver, as represented by figure 10, numerous granuloblasts and hemoblasts 

 may be seen within the lumen of the vessels. It is remarkable that they appear 

 first in the capillaries and veins, and only much later do they become conspicuous 

 in the arteries also. The hemoblasts and granuloblasts developing from the mesen- 

 chyme around the capillaries apparently find an easy access into their lumina and 

 are carried farther by the veins, in which, even in later stages, they appear in rela- 

 tively greater proportion than in the arteries. 



Mesenchyme of the evened. — While it is still easy to follow the local origin of 

 the hemoblasts and the granuloblasts in the liver, especially around the larger 

 vessels, the study of this process in the adrenal is obscured by the extreme density 

 of the tissue proper of the organ and by the scarcity of the mesenchyme. The 

 strands of the epithelial tissue, as seen in figure 6, are separated by large capillaries 

 and groups of chromaffin or phaochrom cells, and by sympathetic ganglion cells. 



