140 INDUCTION AND ORGANISATION 



branchial pouches, is formed on each side by the walls of the 

 gut. These endodermal pouches come into contact with the 

 ectoderm, and at these points branchial grooves are formed by 

 the ectoderm. Later the grooves break through into the pouches, 

 thereby forming the gill slits. The branchial arches separating 

 the slits are lined with endoderm on the inner side. Externally, 

 they are initially covered only by the ectoderm, though later 

 the endodermal epithelium grows out under this ectoderm. 

 Within each branchial arch there is a band of mesoderm, which 

 produces the muscles, and some mesenchyme originating from 

 the neural crest, which forms the cartilaginous skeleton of the 

 branchial arches. Branched external gills grow out from the 

 outer surfaces of the hindmost three pairs of branchial arches. 

 They consist of epithelium, filled with mesenchyme. 



The formation of these gills was found to be due to induction 

 by the endoderm, the former archenteron wall. If gill region 

 endoderm was grafted under the ectoderm of the flank, gills 

 were formed there (Soveringhaus, 1930). Here, too, it appears 

 that the gill-forming potency initially extends over a large area, 

 its intensity decreasing from a central maximum toward the 

 periphery. As long as the graft is not too small, a normal 

 set of gills develops from any part of this "gill- field" on trans- 

 plantation. Two gill-fields grafted side by side fuse into one, 

 and produce one normal set of gills, provided that both were 

 oriented in the same way (Harrison, 1921). The gill endoderm 

 is not only responsible for the origin of the gills, but it also 

 determines their polarity. This was proved by inverting the 

 endoderm, when the resulting gills also were inverted. More- 

 over, the endoderm determines the specific form, size and 

 pigmentation of the outgrowing gills. This was proved by 

 heteroplastic transplantations between species of the genus 

 Ambly stoma (Harrison, 1927). The ectoderm, however, is not 

 altogether without influence on gill development. Rotmann 

 (1931-35) exchanged the gill-region ectoderm between Triton 

 taeniatus and cristatus, and between axolotl and Triton. He 

 found that the gill-ectoderm determines especially the speed 

 of gill development during the earlier stages (Fig. 51). It was 

 also shown that it influences the size of the gills. 



