280 EDUARD UHLENHUTH 



of series XXVI, which were grafted from the pimctatum larvae 

 to the Tigrinum larvae. They showed themselves as normal 

 by the fact that the breaking up of the yellow ring occurred in a 

 normal way, and as we have selected only this phenomenon for 

 our conclusions, it is not a matter of importance what the con- 

 dition of these grafted eyes w r as in regard to certain other his- 

 tological details. Nevertheless it might be mentioned here 

 that histological examination of the eyes proved that even 

 several months after the breaking up of the yellow ring most of 

 the structures and in some eyes all of the structures were still 

 perfectly normal. Cones and rods were present and unchanged 

 and the stroma iridis, which is the place of the phenomenon in 

 question, showed no divergence from the normal structure of 

 this organ. The only change observed is that the cornea is 

 overgrown by one or more epithelial layers. The course of 

 this process, however, can be followed macroscopically under the 

 binocular. It did not begin before the ring was broken up and 

 since the epithelial layers are translucent in the beginning and 

 can be kept so for a long period by being kept wet, examination 

 of the iris is still possible after the epithelium of the skin has 

 commenced to grow over the cornea. 



3. Concerning Weigl's opinion that metamorphosis of the 

 skin is a process of self differentiation, we do not believe that the 

 results of his own experiments justify such an assumption. In 

 fact it seems that they would appear rather confusing and lacking 

 in uniformity if metamorphosis actually were a process of in- 

 dependent differentiation. 



Our experiments on eye grafting have demonstrated that the 

 age of the host as well as the age of the graft — or more correctly, 

 the stage of development of host and graft — are important con- 

 siderations inasmuch as the metamorphosis factor is not carried 

 into the eye before the host has reached a certain stage of de- 

 velopment. If the organ be taken from an individual at this 

 stage, it would of course develop independently from its new 

 host. On the other hand, very young organs for some reasons 

 respond to the action of the metamorphosis factor only after 

 they have been grafted to the new host a certain length of time. 



