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DEPARTMENT OF EMBRYOLOGY.* 



George L. Streeter, Director. 

 CYTOLOGY. 



It may be well to explain that in tissue-cultures individual growing 

 cells from a bit of explanted embryonic tissue, in making their way 

 into the surrounding medium of clear fluid, stretch themselves out 

 flat and very thin on the lower surface of the glass cover-slip, under 

 which conditions they can be readily examined in the living state 

 with high magnification. This method, which has been perfected in 

 large part by Professor W. H. Lewis and Mrs. M. R. Lewis, of this 

 laboratory, has enabled us to study the cytoplasm and finer anatomy 

 of the cell more intimately than was before possible, and at the same 

 time to introduce experimental procedures and observe the behavior 

 of cells in altered environments and under the influence of various 

 stains and reagents. It is quite likely that the essential structure 

 of a growing cell in tissue-culture is the same as that of similar cells 

 growing normally in the embryo, but the shape of the cell, owing to 

 the flattening-out tendency, usually departs considerably from its 

 natural outlines. On this account we have had much difficulty in 

 identifying the unfamiliar forms exhibited by the cells in cultures. 

 Sufficient progress, however, has now been made in the study of their 

 appearance and behavior to make possible a description of the charac- 

 teristics of the primary cell types, as seen under these conditions. 



Characteristics of Cell-Types in TissuE-CuLTrRES. 



It has been found by Professor Lewis that each cell which migrates 

 out of the explant onto the coverslip does so in a manner peculiar to 

 its type. The blood-cells and clasmatocytes pursue very irregular 

 paths, the individual cells retaining their complete independence and 

 rarely adhering together to form a definite pattern. On the other 

 hand, ectodermal and endodermal cells, which always migrate out in 

 the form of a membrane, adhere to their neighbors in more or less 

 even lines. Intermediate between these two extremes are the mesen- 

 chymal, endothelial, and smooth-muscle tissues, in which the cells 

 tend to adhere to one another by their processes rather than b}^ their 

 borders, thus forming loose reticuli, the pattern in each case being 

 characteristic for the respective cell-type. Differing from all of these 

 are the characteristic outgrowths of long, multinucleated strands 

 from striated muscle and the long, slender nerve-fibers from the sym- 

 pathetic and central nervous systems. Both the muscle-strands and 

 the nerve-fibers have a tendency to form anastomising plexuses, 

 those of the nerve-fibers being elaborate and complicated. Once 



♦Address; Johns Hopkins Medical School, Baltimore, Maryland. 



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