246 EDUARD UHLENHUTH 



velopment of the background in other animals the spots would 

 have been better developed than in the animals referred to. 



The four stages which we have found during the development 

 of the skin are (1) even yellow brown, (2) network, (3) separa- 

 tion of network and (4) black; the third one, because of its 

 simultaneity with the general metamorphosis of the whole 

 animal, has been considered to directly correspond with the 

 metamorphosis of the skin. But since the rate of development 

 of the other stages and particularly also of the second stage 

 has been found subjected to the same laws as that of the third 

 stage, there will be no need of emphasizing this artificial dis- 

 tinction when we discuss the general laws to which the develop- 

 ment of the skin appears to be subjected. The second and the 

 third stage together constitute the phenomenon of the working 

 out of one typical characteristic of the adult, i.e., the yellow 

 spots, which is the first step of this stage. 



B. EYE 



In the eye also certain changes of its pigmentation were used 

 as a mark of the entrance into metamorphosis. 



In the beginning of the larval period the eye shows, when 

 examined with the naked eye, a continuous rather light yellow 

 ring, which corresponds with the iris and contains no black 

 pigment. The color of this ring darkens with increasing age 

 but no black pigment forming interruptions between the yellow 

 parts of the ring are visible to the naked eye. Some time be- 

 fore the larvae leave the water the ring becomes narrower and 

 finally forms a distinct and narrow but still continuous line 

 around the pupil. Several days after the animal has left the 

 water, the ring undergoes a distinct and very sudden change ; the 

 yellow line breaks up into numerous yellow and black dots. 

 After this there occurs a very gradual change, which stretches 

 over a long period; while the black dots become larger, the num- 

 ber of yellow dots decreases until finally pupil and iris form one 

 black circle on the outer margin of which a small number of yel- 

 low dots are scattered. In some individuals no dots remain 

 while in others yellow dots are present for several months. 



