GROWTH 



347 



entire layer, nor even all the dead tissue. Only the hardened, 

 more superficial part separates from the deeper, more flexible, 

 layers. In some cases (Reptiles) a specially constructed layer 

 of cells forms a cleavage plane. The process is comparable to 

 the ecdysis in Arthropods, except that here we have to do with 

 dead cells instead of formed substances. 



734. The method of growth of the bones varies greatly. In 

 the smaller bones with simple form, the process is not specially 



FIG. 215. The carapace of the diamond back terrapin, Malaclemmys palus- 

 tris. Note the concentric lines of growth in the horny plates. X 1/2. 



noteworthy, but with the "long" bones and those of com- 

 plicated figure, the enlargement of the bone, and at the same 

 time maintaining its form, is often a complicated process. For 

 example, the skull of the adult is practically a single piece. 

 This condition could not have been reached by the addition of 

 layers of bone to the surface, since this would not provide for 

 the growth of the brain unless, at the same time, the cavity of 



