220 STRUCTURE AND GROWTH 



In the first place, it will be remarked that in equal bulks 

 of the tissue there is present a larger quantity of bioplasm 

 in the lower part, a, which is close to the vessels, than in 

 the upper part, c, which is a long distance from the nutrient 

 surface, and that the converse of this holds as regards the 

 formed material to which this tissue owes its properties and 

 physical characters. Secondly, it will' be noticed that the 

 individual masses of bioplasm increase in vsize till they 

 arrive at a point about half way (at b) between the deep 

 aspect and the surface ; while from this point to the surface 

 they diminish (c) and thirdly, that the distance between 

 them increases on account of the increased formation and 

 accumulation of formed material. But by the time the cells 

 have reached the free surface, the distance between the 

 several masses of bioplasm is reduced again, in consequence 

 of the drying up and consequent shrinking and condensa- 

 tion of the formed material. 



The changes which each individual cell or anatomical 

 unit passes through may now be considered. At the deep 

 aspect near the nutrient surface are masses of bioplasm 

 embedded in a soft, mucus-like, and, as yet, continuous 

 formed material, a. The masses of bioplasm divide, and 

 each of the resulting masses becomes invested with a thin 

 layer of the mucus-like matter. In this way, the elementary 

 parts, anatomical units, or cells, multiply in number, to com- 

 pensate for the loss of those old cells which are gradually 

 removed from the surface.* Each mass of bioplasm in- 



* The description here given is not strictly accurate, inasmuch as 

 the new masses of bioplasm do not all move in a direction towards the 

 surface. Some tend in the opposite direction, towards the subcuticular 

 tissue, but this need not be discussed here, as it would complicate the 



