CELL CONSTANCY IN THE GENUS EORHYNCHUS 257 



high degree of constancy in the number and the arrangement 

 of its component cells. 



Any tissue, the cells of which retain the power of continued 

 multiplication after they have become differentiated, is said to 

 possess the power of physiological regeneration. On the basis of 

 the presence or of the absence of this property, Bizozzero has 

 distinguished three different types of tissues within the human 

 body. Morgan ('01, p. 128) quotes him as follows: 



1. Tissues made up of cells which multiply throughout life, as the 

 parenchyma cells of those glands which form secretions of a definite 

 morphological nature; the tissues of the testes, marrow; lymph glands, 

 ovaries; the epithelium of certain tubular glands of the digestive tract 

 and the uterus; and the wax glands: 



2. Tissues which increase in number of their cells till birth, and only 

 for a short time afterward, as the parenchyma of the glands with fluid 

 secretions, the tissues of the liver, kidney, pancreas, thyroid, connective 

 tissue, and cariilage; 



3. Tissues in which multiplication of cells takes place only at an early 

 embryonic stage, as striated muscle and nerve tissues. In these there is 

 no physiological regeneration. 



There is no need of limiting this classification to the tissues of 

 the human body for the general principle applies equally well to 

 the tissues of any animal. The direct dependence of the idea of 

 cell constancy upon the conditions set forth in the second and 

 third groups of this scheme is strikingly evident without the need 

 of further discussion. Those_ tissues which in most animals 

 possess the power of physiological regeneration have been elimi- 

 nated or profoundly modified in all forms displaying cell constancy. 

 Two of the most obvious factors involved in accomplishing this 

 reduction of inconstant tissues are modification of organs through 

 degeneration and complete elimination of parts through adapta- 

 tion to parasitism. Thus in the Acanthocephala adaptation to 

 the parasitic habit has been so complete that all traces of the 

 glands usually associated with the processes of metabolism are 

 wanting. With the elimination of this great group of organs there 

 is presented a condition most favorable for the development of a 

 permanent, fixed, relation of the component parts of the body. 

 In the genus Eorhynchus this finds expression in a surprisingly 



