1913] Ilolmcs: Ectodcrmic Epithelium of Tadpoles 157 



spreading out into a layer but one cell thiek. Mitoses did not 

 appear until a considerable amount of spreading had been 

 accomplished. Although the outwandering cells commonly 

 became more or less flattened, measurements showed that they did 

 not increase appreciably in the extent of their surface. Movement 

 occurred as a rule without pseudopods; nevertheless the cells 

 changed considerably in form. No evidence could be found that 

 the movements were determined by cellular contractility. Oppel 

 compared the cells to partly filled sacks of inelastic material 

 which could change their outline without varying the extent of 

 their surface. The form changes are presumably correlated with 

 the movements of the cells, and these movements, while made in 

 response to external stimuli, are attributed to internal factors; 

 but the real modus operandi of epithelial movement Oppel fails 

 to make clear. Epithelial movement, which Oppel makes a 

 distinct category of activity, has, according to him, the following 

 characteristics : 



' ' Die Epithelbewegung vollzieht sich nicht unter Aussenden 

 von schmaleren oder breiteren Fortsatzen (sog. Pseudopodien), 

 wie dies bei der Leucocytenbewegung der Fall ist. " 



"Die Epithelbewegung geht nicht mit Isolienmg (absolute 

 Selbstiindigkeit) der einfach einzelnen Zelleu einher, sondern sie 

 fiihrt im Gegenteil zu einer flachenhaften Beriihrung gleichar- 

 tiger Zellen (Cytarme). 



"Die Epithelbewegung ist oft eine Massenbewegung. Gauze 

 Zellgruppen bewegen sich miteinander infolge von Reizwirkung, 

 jedoch mit steter Lageveranderung der einzelnen Zellen unter- 

 einander. " 



"Die zur typischen Gestaltung fiihrende Epithelbewegung 

 bleibt im wesentlichen auf die Oberflache bindgewebiger Flachen 

 besehrankt (Desmophilie)." 



Lewis and Lewis (1912) have recently described memlirane 

 formation in the tissue of the chick embryos cultivated in various 

 artificial media. These membrane-like outgrowths often spread 

 out for a considerable distance into the surrounding medium. 

 The cells at the edges of the outgrowth in many cultures were 

 irregular in outline and sometimes ended "in long irregular 

 processes with an enlarged fimbriated end". Many of the cells 



