APPENDIX 



I . Exogastrulation in Amphibia. 



The work of Hohfreter (Arch. Entivmech. cxxix, 1933, p. 669; 

 Biol. Zentralbl. Liii, 1933, p. 404) on this subject only appeared after 

 this book was in page proof. It is, however, so important that we 

 have decided to summarize it in an appendix ; and have taken the 

 opportunity of adding some other points that had been overlooked. 



Head muscles 

 MoutK endoderi 



Fig. 213 



Exogastrulation in Amphibia (axolotl). Diagrams showing {a, b) the mass- 

 movements of the organiser-region and endoderm ; c, the structure of an exo- 

 embryo. Note wrinkled amorphous epidermis, exogastrulated endo-mesodermal 

 portion inside-out. (From Holtfreter, Biol. Zentralbl. liii, 1933.) 



Holtfreter discovered that by the simple procedure of removing 

 the early blastulae of axolotls from their membranes and placing 

 them in Ringer solution of about 0-35 per cent, strength they could 

 be made to exogastrulate — i.e. the presumptive endoderm and 

 mesoderm is evaginated instead of being invaginated, leaving the 

 presumptive ectoderm as a hollow sac. Stages in the process are 



HEE 31 



