166 University of California Puhlications in Zoology [Vol. 11 



are unable to cheek their "Wanderlust" when an opportunity 

 presents itself for migrating over a greater extent of surface. 

 The individual cells, as we have seen, manifest a tendency to 

 spread out so as to bring as large a part of their surface as 

 possible in contact with the substratum. And it seems very 

 probable that the extension of strands and sheets of ectoderm is 

 brought about by the same kind of activity that occasions the 

 spreading of the individual cells. The method by which the 

 ectodermie epithelium of tadpoles comes to spread out over 

 surfaces is probably different, therefore, from that which Oppel 

 concludes is followed by the epithelial cells of mammals. 



When conditions become unfavorable the cells tend to assume 

 a more rounded form. This tendency is probably responsible for 

 the shortening and thickening of strands that were several times 

 observed, and the drawing in of cords of cells. I have been able 

 to cause a contraction of strands of cells by touching the cover 

 glass above them with a warm needle. The same treatment 

 causes the individual cells to draw in their pseudopods. Not 

 only do epithelial cells respond in this manner to thermal stimuli, 

 but in the more irregular cells of the mesenchyme the response 

 is more decided. In pigment cells the retraction of the processes, 

 while evident, is less pronounced. A sharply defined band of 

 strong light focused on the epithelial cells had very little effect 

 upon their activity. 



Several experiments were undertaken in order to ascertain 

 if the epithelial cells in the cultures actually increased in number. 

 Small pieces of tissue were placed in serum, and after a number 

 of cells had become free from the piece, a small part of the 

 serum was drawn into a very fine pipette and transferred to a 

 larger drop of the same substance which was sealed up in the 

 usual manner. In this way a small number of cells were usually 

 transferred and these were carefully counted and recorded. 

 They were then recounted at intervals of one or more days. In 

 several cases a part of the cells was transferred after two or three 

 days to fresh serum or plasma, and counted at different times 

 for some weeks. In many of the preparations it was found that 

 in the first two days the number of cells had considerably 

 increased ; in a few instances it had doubled ; and a second 



