HISTOLOGY OF THE THYMUS 307 



best growths obtained took place in the plasma of very large, 

 and presumably old frogs. 



The influence of mechanical factors has been repeatedly em- 

 phasized by workers with this method. The direction of the 

 growth, as well as the configuration of individual cells, is deter- 

 mined in large part by the direction of the fibrin threads which 

 act as support. 



In a successful preparation, one may observe the following 

 sequence of events. In the teasing of the fragment, many small 

 thymic cells are often separated from the main fragment, and 

 are then distributed far into the surrounding plasma. The cen- 

 tral bit of tissue soon becomes surrounded by a fringe or halo 

 of isolated cells. Most of these are cells of the lymphoid type, 

 but one may identify coarsely granular cells (eosinophiles?) and 

 scattered myoid cells, the appearance of which in the living has 

 already been described. After a few hours, the isolated small 

 cells sink to the bottom of the plasma. Some however, remain 

 adherent to the cover-glass and migrate out to a considerable 

 distance from the main fragment. As regards the character of 

 the amoeboid activity of the small cells, it is, as Hammar (10) 

 has pointed out, identical with that of the small lymphocytes of 

 the blood, as described by Jolly (11), Askanazy (12), Meves (13) 

 and others. Lobe-like hyaline psuedopodia are extruded and 

 retracted, first from one portion of the protopl-asmic margin, 

 and then from another. Often these pseudopodia are long and 

 hair-like, and a number of such delicate filamentous processes, 

 which may reach a length several times the diameter of the thy- 

 mus cell, project from different points of the circumference. 

 \\Tien there are active movements of progression, the pseudo- 

 podia of this type seem to be dragged astern like a rudder. The 

 entire cell may become constricted into two lobes and appear to 

 be about to divide, the nucleus changing its contour somewhat 

 with the changing contour of the plasmatic prolongations. 



The amoeboid activity of certain of the small cells may be 

 maintained for six days or more. Especially at the periphery 

 of the fringe of isolated cells at the bottom of the plasma, one 

 finds many active cells. If a single cell be observed for a pro- 



