498 APPENDIX 



5. Development of the amphibian mouth (see also p. 179). 



The recent work of Stroer (Arch. Efitwmech. cxxx, 1933, p. 131), 

 on mouth-development in Amblystoma, may be referred to as an 

 excellent example of detailed experimental analysis. He finds, by 

 means of grafting experiments, that the mouth-region is composed 

 of both ectodermal and endodermal portions, of which the ecto- 

 dermal alone is capable of forming teeth. The presumptive mouth- 

 ectoderm is dependent on underlying endoderm for the realisation 

 of its potencies : without this, it develops into epidermis. Ectoderm 

 from the ventral region of the abdomen grafted in place of the 

 presumptive mouth-ectoderm does not react with the underlying 

 endoderm to produce a mouth (contradicting Adams; see p. 179). 

 On the other hand, pieces of the mouth-inducing region of the 

 endoderm (anterior wall of fore-gut) introduced into the blastula 

 may produce a mouth in interaction with the ectoderm of the heart 

 region. Thus we do not know the limits, either in space or time, of 

 the ectoderm-field capable of reacting to form mouth-epithelium. 

 When presumptive mouth-endoderm grafted into the blastula does 

 not succeed in inducing the ectodermic portions of a mouth, it 

 develops into endodermic portions only (portions of buccal cavity, 

 pharynx, oesophagus, with taste-buds, but without teeth). 



An interesting point is that if only a small piece of ventral ecto- 

 derm is grafted into the presumptive mouth-region, it is caught up 

 in the invagination process carried out by the remaining presump- 

 tive mouth-ectoderm and apparently *' infected" with its qualities, 

 for it then differentiates into true mouth-epithelium. 



Another point is that the determination of the presumptive 

 mouth-ectoderm to produce teeth takes place rather earlier than 

 its determination to become differentiated mouth-epithelium. 



The only region capable of inducing mouth-formation is the 

 anterior wall of the fore-gut. This was proved by implanting 

 pieces of various regions of the developing egg into blastulae. 

 Pieces of anterior neural plate or neural fold have no mouth- 

 inducing capacity. Neither neural tube nor neural crest material 

 is necessary for tooth-formation, so that the mesodermal portion of 

 the teeth must be derived not from mesectoderm but mesendoderm. 

 Taste-buds are produced by pure endodermal grafts (see p. 487). 



