44 



HISTOLOGY 



cells that do not, in any way, touch the outer surface. These inner cells 

 have been differentiated apart from the epithelial cells which now lie 

 between them and the outer world, and where the two touch is 

 found a surface that is the inner end of the epithelial cell (Fig. 48, C). 

 Repeated divisions now reduce the size of the cell in proportion to the 

 size of the organism until we find the condition seen in Figure 48, D, 

 where the epithelial cells form a flat, even sheet of tissue covering every 

 part of the body. Many other cells have divided off from the outer or 

 epithelial cells and have been added to the inner mass of cells. Some 

 few epithelial cells (i.e. cells touching the outer surface) have been 

 crowded out of the outer row and pushed into the inner mass, where they 

 can be distinguished by the pigment lying in what was formerly their 



FIG. 48. Four stages in the cleavage of the ovum of a toad, Hyla pickeringii. 



outer end. Other inner cells have been crowded by the inner growing 

 mass out into the epithelium, a rarer case than the former. Out of all 

 this change only such cells as maintain a position on the outer surface 

 become the epithelial cells. Their use is determined by their position, 

 not by any structure or predetermined feature that we are able to 

 demonstrate. After the period of differentiation, when the epithelial 

 cells have attained their final characteristics, they can no longer take 

 up the position or function of a mesoderm cell; nor can the latter 

 become an epithelial cell in the higher forms. In the lower forms these 

 exchanges of position or function are often possible and take place 

 during regeneration. 



Each epithelial cell has a flat, even, outer end that unites with the ends 

 of its surrounding neighbors in conforming to the surface of the body. 

 The inner ends of the cells, at first very irregular (Fig. 48, C), soon become 



