APPENDIX 489 



isolated ectoderm (see figs. 13 and 219). This appears to be due 

 largely to the formation of mesenchymatous vesicles containing 

 fluid, which produce normal tension in the ectodermic vesicle. 





Fig. 219 



Organisation of epidermis, a, wrinkled, irregular epidermis of axolotl exo- 

 embryo not underlain by organiser; b, two-layered epithelium induced by pre- 

 sence of underlying connective tissue and blood-cells. (From Holtfreter, .4ir/z. 

 Entwmech. cxxix, 1933.) 



Neural formations are never induced in such conditions, but are 

 always formed if any of the ectoderm comes to be underlain by 

 chorda-mesoderm. If only a narrow portion of chorda-mesoderm 



