86 



NERVOUS SYSTEM OF VERTEBRATES. 



Even before this theory had been expressed the cell theory 

 itself was being severely criticized and it may be said that since 

 the year 1890 it has undergone important modifications. Although 

 we may still speak of many- celled animals and may regard cells 

 in a general way as units of structure, we can no longer consider 



Fig. 42. — A portion'of the subepithelial nervous plexus in the palate of the frog. 

 From Prentiss. 



cells as unit organisms which join in the formation of an organism 

 of a higher order. Cells are not always completely bounded by 

 cell walls and in many cases adjacent cells are directly continuous 

 with one another by means of strands of protoplasm. Moreover, 



