DEVELOPMENT OF THE TADPOLE. 71 



be seen more clearly as we proceed with their life-history. 



Having seen it stated that tadpoles would not develop if kept 

 in the dark, a few experiments were made on this point. Embryos 

 of the age shown in Plate VII. were placed in a perfectly dark jar 

 on March 7th, and kept side by side with those developing in the 

 ordinary glass tanks, so as to be exposed to the same external 

 conditions, with the exception of the exclusion of light. 



At the same time, another batch from the same lot was placed 

 in a dark cupboard at the top of the house, where the temperature 

 was frequently below freezing and never above 40? F. These 

 were in a glass jar, and were, of course, under very different con- 

 ditions to those kept in a room where the maximum temperature 

 might sometimes be 55° to 60° F., but whose minimum never 

 went below 40°. On March i8th it was found that the develop- 

 ment of the animals kept in the jar had proceeded equally with 

 those kept in the glass tanks, the only appreciable difference being 

 that those kept in the dark were on the whole a shade lighter in 

 colour than those kept in the glass tanks. It was, however, very 

 different with those in the cupboa'-d, many of which were dead 

 and all much arrested in development. All were dead by March 

 28th : several others being transferred from the dark jar to the 

 cupboard, all died by April 5th. Suspicion was now cast on the 

 glass vessel in which the animals were kept being the real cause of 

 this wholesale destruction, so that another was substituted identical 

 in every respect with that kept in the warmer room in the light. 



A number was then transferred from the dark jar to the new 

 glass vessel and removed to the cold dark cupboard. By April 1 1 

 five only remained, and by May 4th, barely a month after they 

 were placed in the cupboard, all had died. As there were no 

 apparent circumstances connected with the surroundings or atmo- 

 sphere of this cupboard to account for the death of all these 

 animals, the only conclusion to which I could arrive from this 

 series of experiments was that whereas development proceeded as 

 well in the dark as in the light, provided the temperature did not 

 fall below 40° F., yet on the removal of animals which were 

 l^reviously developing under these conditions to a lower tempera- 

 ture in darkness, development was arrested, and death invariably 

 ensued. 



