DEPARTMENT OF EMBRYOLOGY. 81 



times a very slight disturbance may produce the withdrawal of processes and 

 the rounding up of cells. It is clear that the physical factors of cohesion and 

 surface tension or capillary attraction are constantly at work in altering the 

 form of the cells and their processes, and it is in this direction that we must look 

 for an explanation for the characteristic morphology of the different tissues. 



Characteristics of Cell-Types in Growing Tissues. 



In my last report I mentioned the progress that had been made by Dr. 

 Lewis in the identification of the characteristics of the primary cell-types, as 

 seen in cultures of growing tissues. During the past year he has completed a 

 careful study of endothelial cells as they migrate out from embryonic chick 

 liver-tissue. The liver at 96 hours' incubation presents the advantage of 

 containing only two types of cells, the liver-cells proper and the endothelium 

 of the sinusoids. No difficulty is encountered in distinguishing these two 

 varieties of cells, the first growing out as a membrane and the other as a loose 

 reticulum. Observers, working with the ordinary explant, have in the past 

 had difficulty in identifying reticular radiating outgrowths and could not be 

 sure whether, in addition to mesenchyme and fibroblasts, there were not some 

 mesothelium and endothelium present. This uncertainty has in large part 

 been removed by the detailed description and photographs which Dr. Lewis 

 has published of known growths of endothelium. 



In collaboration with Dr. L. T. Webster, Dr. Lewis has completed a study 

 of wandering cells, endothelial cells, and fibroblasts in cultures from human 

 lymph-nodes. The wandering cells were particularly interesting. After 2 

 or 3 hours' incubation they migrated from cultures of both normal and (in 

 greater number) chronic inflammatory glands and were actively ameboid and 

 phagocytic. These cells differ in size from the endothelial cells, but in struc- 

 ture and behavior they closely resemble them. Frequently transitional forms 

 are found and it seems quite probable that the wandering cells are derived from 

 endothelial cells. 



Dr. M. J. Hogue has made a comparative study of living fibroblasts from 

 the embryonic chick and salt-water amebse obtained from the digestive tract 

 of the oyster. By introducing the amebse into the cultures of growing tissue- 

 cells, it was possible to observe the reaction of the two lands of cells to various 

 vital dyes and pigment granules. In structure and behavior the ameba 

 resembles the fibroblast. It is, however, not quite so large, the cytoplasm is 

 denser, its outline is more definite, and it moves more rapidly. The presence 

 of non-pathogenic bacteria is not injurious to the ameba, but is harmful to 

 tissue-cells. On the other hand, amebse are much more sensitive to vital 

 dyes. When melanin pigment granules were put into hanging drops with the 

 amebse they were taken up by some and not by others, and it may be that the 

 physiological condition of the ameba is a factor in this phenomenon. When 

 an ameba takes in granules it does so in the same manner it takes in its food : 

 it appears to push against the granule, whereupon the latter penetrates the 

 ectoplasm. The granule must therefore be directly in the path of the advanc- 

 ing ameba. After the granules have entered the endoplasm they circulate 

 freely through it. There is some tendency for them to clump, in which case 

 a vacuole may form around them. These vacuoles are heavy and lag behind 

 in the posterior part of the cell, and on coming in contact with the edge of the 



