284 S. J. HOLMES 



amoeboid processes may be seen to give way suddenly as if they 

 could no longer resist the tension of the cells behind. The free 

 margin then becomes thickened and rounded, and the cells take 

 on a quite different form. When one part of the hyaline border 

 gives way the adjacent parts follow; the retraction, however, may 

 soon stop, or it may spread widely according to various circum- 

 stances. At the beginning of the contraction the pseudopods 

 may be stretched out considerably, as the cells behind tend to 

 draw away, before they become loosened from their attachment. 



A characteristic of these epithelial outgrowths quite as striking 

 as their extreme contractility is their extraordinary sensitive- 

 ness to slight stimulations. Bringing the slides from a cool 

 place to the stage of the microscope where the temperature is a 

 few degrees higher usually causes a retraction of some part of 

 the marghi of an epithelial outgrowth. The process of con- 

 traction was observed a great many times, and in fact it usually 

 happens to a certain extent whenever slides are brought from a 

 cool place for examination. It is probably the transition to a 

 warmer environment rather than the higher temperature per se 

 that causes the contraction, because preparations kept at room 

 temperature form equally great extensions of epithelium, and in 

 many cases the sheets of ectoderm which become contracted 

 when exposed to a higher temperature subsequently extended 

 again while the higher temperature remained unchanged. Con- 

 traction of any part of the sheet of ectoderm may be iniated 

 by placing the point of a warm needle above the region in question. 



That the cells are responsive to a mechanical stimulus was 

 shown by touching the margin of the extension with a fine glass 

 rod. Local contractions uniformly followed. Whenever a prepa- 

 ration was washed in Ringer's solution preparatory to furnishing 

 it with a new supply of nutrient gelatine, the epithelial extension 

 shrivelled up to a small fraction of its previous dimensions. By 

 placing a drop of Ringer's solution upon the preparation the 

 epithelium is caused to contract as soon as the solution diffuses 

 into contact with it. Whether the contraction is due to a slight 

 osmotic effect of the Ringer's solution, or to the stimulating 

 influences of the salts is uncertain. 



