XVI 



THE INFLUENCE OF LIGHT ON THE DEVELOPMENT 

 OF ORGANS IN ANIMALS 1 



I. EARLIER EXPERIMENTS 



Two GREAT series of experiments which Nature herself 

 has made are at our disposal for answering the question as 

 to what effect light has on the development of animals; 

 namely, the intra-uterine development, and the development 

 of animals living in caves. The fact that intra-uterine 

 development goes on in complete darkness proves that the 

 formation of the embryo and its organs, histological differ- 

 entiation, and considerable growth can occur and continue 

 for a long time in the absence of light. As far as animals 

 living in caves are concerned, some of them differ from the 

 same forms which live in the light in the development of 

 single organs, such as eyes, antennae, and pigment. It has 

 not, however, as yet been proved that this peculiarity of the 

 cave inhabitants is a direct effect of the lack of light upon 

 their development; but granting that it is the direct result 

 of lack of light, it follows from a summation of the facts in 

 hand that where light has any direct effect whatsoever on 

 development, it evidently makes itself felt only upon the 

 development of individual organs and not upon the develop- 

 ment in general. 



It is strange that, notwithstanding the definiteness of 

 these facts, experimental work on the influence of light on 

 the development of organs in animals has been directed 

 mostly to the question whether light promotes or inhibits 

 development and growth of animals in general. 



i Pfliigers Archiv, Vol. LXIII (1895), p. 273. 



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