262 Experimental Zoology 



the light rays to pass through. Furthermore by light we usually 

 mean sunlight or "white light," which is a composite light made 

 up of vibrations of different lengths that we recognize as colors. 

 One of the most unexpected facts in regard to the influence of 

 light is that in plants light is antagonistic to growth, although 

 light is necessary for the normal existence of all green plants. 

 The paradox finds its explanation in that most of the actual 

 growth takes place at night; while light is a form of energy 

 necessary to fix the carbon of carbon dioxide in the chlorophyll 

 body. Out of this carbon and other things the starch is formed, 

 and starch when transformed into sugar is the food of the plant 

 supplying the material for growth. Davenport has pointed 

 out "that it is noteworthy that embryonic tissue, and indeed the 

 entire embryonic individual, is usually sheltered from sunlight. 

 In animals the embryo is sheltered in the darkness of the maternal 

 body ; in birds and reptiles the egg shells are more or less opaque, 

 and, moreover, the whole egg is usually hidden from light." 

 Blanc (1892) has shown that the development of the hen's egg 

 is much retarded if subjected to light. 



Many animals hide their eggs, but whether this has only the 

 advantage of concealment from enemies, or of placing them out 

 of the light, we do not know. On the other hand, pelagic fish eggs 

 are exposed during the day to the full light, and yet develop nor- 

 mally. Maupas found that ciliate infusoria multiply equally 

 fast in the light and in the dark. Experiments have also been 

 made with tadpoles. Edwards in 1821 stated that tadpoles 

 would not develop at all in the dark. Later Higgenbotham 

 (1850 and 1863) and Macdonnell showed that they grow at the 

 same rate in light and in the dark. Yung also made a study 

 of this problem. His results are given in the following table. 

 In each case "typical" tadpoles were measured in each lot. 



