184 Studies in Animal Behavior 



the blood vessels of the yolk sac, thereby producing 

 a certain color pattern characteristic of the species, 

 a result probably due to some tropism, either a 

 chemotaxis toward oxygen, or a thigmotactic re- 

 action to the walls of the blood vessels. 



Both connective tissue cells and pigment cells when 

 isolated tend to withdraw their processes un- 

 der unfavorable conditions and assume a spherical 

 form. Too high a temperature tends to cause this 

 response, and the same reaction commonly results 

 when the culture medium becomes contaminated by 

 the accumulation of metabolic products. An Amoeba 

 under similar conditions behaves in essentially the 

 same way. 



As is well known, the cells of epithelial tissues 

 are almost always arranged in definite layers one 

 or more cells thick. Such tissues are found cover- 

 ing the entire surface of the body, and lining all 

 the inner surface of the alimentary canal and its 

 various appended organs; epithelium forms the in- 

 ner lining of the body cavity, the blood vessels and 

 lymphatics; in fact, in almost every situation in which 

 a free surface occurs it is covered by a layer of this 

 tissue. If for any reason a part becomes denuded 

 of its coating of epithelium it is usually rapidly cov- 

 ered again by extensions of this tissue from con- 

 tiguous areas. There have been various opinions 

 as to just how this extension is brought about. Sev- 

 eral investigators have concluded that it is mainly 

 effected by the migration of epithelial cells, and the 



