250 EDUARD UHLENHUTH 



both sides of the lateral 'os occipitale.' Among 83 -individuals 

 for instance, which were examined in this respect, only 16.9 

 per cent did not have any spot on the right side (only this side 

 was counted) of the caudal end of the head, while 83.1 per cent 

 of the animals had developed one or more spots. Therefore, if 

 the skin of the head is used, the chance of obtaining yellow spots 

 in the graft is about 83 per cent and one is also prepared at the 

 time of metamorphosis to look for these spots on the rear end 

 of the skin graft, provided that the skin be grafted in such a 

 way that its front and rear correspond to cranial and caudal 

 respectively of the new host. In the experiments the actual 

 number of grafts which developed spots on the rear end was 

 much smaller than 83 per cent, the reason for this being that 

 when the grafts were taken from the larvae, they did not always 

 include the area where the spots mentioned above are located. 



The technique of operation was exactly the same used in 

 former experiments and may be learned from previously pub- 

 lished papers on these experiments. 3 It should be mentioned 

 however, that while front and rear end of the graft were left 

 unchanged, each piece of skin was turned upside down on its 

 new host, as can be seen from the sketch in figure 1. In the 

 first six experiments the animals were kept in an icebox at 0° to 

 3°C. before operation. All other animals were kept in a dark 

 cool-room at a temperature of 14° to 18°C. before operation. 

 Immediately after operation each animal was placed in a petri 

 dish, the bottom of which was covered with moist filter paper 

 and a thin layer of water, and placed in a dark incubator at 16° 

 to 18°C. for about twelve to sixteen hours. Then each one was 

 put into a white glass jar holding about 2000 cc. ; the jars were 

 filled with tap water and placed in a dark cool-room at an 

 average temperature of 15°C. where the animals were kept 

 permanently. 



When the animals should be taken out of the water they show 

 a rather definite and uniform behavior. First they come to the 

 surface of the water in order to change the air contained in the 



3 E. Uhlenhuth, 1912, p. 735. 



