GROWTH 351 



further toward the two ends. After a time new centres of 

 ossification occur at each end. These form bony discs, which 

 are later separated from the shaft only by a thin seam of carti- 

 lage. In this seam there are, however, three zones of growth; a 

 middle zone, in which cartilage is rapidly forming, and on either 

 side of this is a zone in which the cartilage is being eroded and 

 replaced by bone. It is thus that the shaft increases in length, 

 while the epiphysis is also increasing in thickness (compare 

 with the growth in the cambium ring). When growth is com- 

 plete the epiphysis unites with the shaft. 



736. This method by which a complicated skeletal figure 

 expands by interstitial growth through the growth of parts is 

 also found among invertebrates. An excellent example is in 

 the test of the sea urchin. 



737. In cases where the proper form cannot be secured 

 through growth by the addition of material to earlier stages, 

 it often happens that parts of the earlier structure are actually 

 removed, so that growth consists in a process of tearing down 

 and building larger. This takes place, for instance, in many 

 gastropod shells which form a thick rounded fold at the mouth 

 of the shell at the close of the seasonal growth period. At the 

 beginning of the next growing season this fold is removed by 

 absorption before the edge of the shell is extended. (Ex. : The 

 queen conch.) This also very often occurs in the development 

 of the Vertebrate skeleton. The central cavity of the shaft of 

 an adult femur, for example, is much larger than it was when 

 the first layer of bone was laid down on the cartilage. Hence, 

 the bone must have been removed at a later period. So also 

 the lower jaw of the embryo, with its complex curvature, can- 

 not be included within the outline of an adult jaw. There 



Fig. 218. — Three successive steps in the growth of the queen conch (Cassia). 

 The thickened Up of the shell ( + ) in ^, is shown in 5 ( + ) partly absorbed and 

 overgrown by the new growth. A new Up (o) is formed after a period of growth 

 and this is again partly absorbed and overgrown {0, in C). In C, nine or ten 

 successive stages of growth may be counted by the remnants of the lips. X 1/2. 



