XVI 
THE INFLUENCE OF LIGHT ON THE DEVELOPMENT 
OF ORGANS IN ANIMALS! 
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, antenne, 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. 
1 Pfliigers Archiv, Vol. LXIITI (1895), p. 273. 
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