^^^ ZOOGENESIS '^'^^^ 



maximum amount of internal living bony structure 

 that can be nourished and maintained by animals of 

 their size. Their skeleton therefore is in a state of 

 very delicate adjustment to their general bodily con- 

 dition. This is particularly true of rapidly grow- 

 ing young. 



Young hard shelled pond turtles kept in a house 

 and deprived of sunlight in spring or early summer 

 show within two days a noticeable deterioration in 

 their skeletons, and within a week the deterioration 

 has become marked. Older individuals of some spe- 

 cies are affected almost as quickly. 



When replaced in sunlight the affected individuals 

 almost at once begin to show improvement, and unless 

 the deterioration has progressed too far soon return 

 to normal. 



Since light is so essential for the well being of the 

 turtles, all turtles have good eyes, and none of them 

 is blind. 



The crocodiles and alligators seem to be almost as 

 dependent upon sunlight for their well being as are 

 the turtles. 



While light in some form and in some degree is 

 apparently essential for all animal life, an excess of 

 light, especially of certain types of light, is distinctly 

 harmful. 



There are two ways of guarding against the possible 

 injurious effect of too much light. In the first place, 

 the possession of a transparent glassy body permits 

 the passage of light through it without injury. We 

 find such transparent bodies in many of the jelly- 



[75] 



