105 



on three different species of frogs (Rana sylvatica, Rana palustris and 

 Rana virescens) and have, I believe, succeeded in showing that after 

 destroying the somatopleure of the limb-region, the limb will, not- 

 withstanding, develop as in a normal embryo. 



The experiments were made on embryos in two different stages 

 of growth: — on very young embryos in which the limb-rudiments 

 had not even begun to appear, as in Fig. 1, and on older embryos 

 in which the limb-rudiments were just beginning to develop, as in 

 Fig. 2. 



Fig. 1. Fig. 2. 



---My.Pr. 



-LBad. 



Fig. 1 represents, diagramatically, the structures seen in cross section in the 

 posterior limb-region of a very young embryo, prior to the appearance of the limb- 

 rudiments. 



Fig. 2 represents, diagramatically, the structures seen in cross section^ in an older 

 embryo, after the appearance of the limb-rudiments. 



In Fig. 1 and 2 the extirpated region is enclosed by lines. 



L. Reg. Region of limb. L. Rud. Rudiment of limb. My. Myotome. My. Pr. 

 Myotome process. 



In these older embryos the limbs appeared in cross section as a 

 small group of mesenchyme - like cells lying within the somatic 

 mesoblast. 



The extirpation was made by burning out the limb-region, on the 

 right side of the body with a hot needle, so that the cells of the 

 somatopleure, from which the limb would naturally be derived, were 

 removed leaving the yolk exposed beneath. In order to leave no 

 room for doubt that the injury had included the limb-rudiment, the 



