26 



EARLY AMPHIBIAN DEVELOPMENT 



Stage as the neural plate. All the remainder of the surface of the 

 embryo is now occupied by presumptive epidermis (figs. 3, 7 and 8). 

 The movements which have brought about gastrulation are 

 therefore also responsible for bringing the presumptive neural fold 

 material into place in preparation for the formation of the neurula, 

 and this, in turn, as will shortly be seen, paves the way for the 

 changes which result in the formation of the tail. 



Fig. 7 



The process of gastrulation in Urodeles, revealed by the movement of intra 

 vitatn stain marks placed on the surface of the blastula, as in a. The marks stretch 

 and move towards the blastopore rim. In b mark i has become invaginated ; in 

 d only mark 4 is left on the surface; the others have become invaginated and 

 passed forwards, forming the gut-wall, and can be seen by transparency through 

 the epidermis. (After Goerttler, Arch. Entwmech. cvi, 1925, modified.) 



Accompanying the processes of displacement and stretching 

 which have just been described, growth also takes place, which pro- 

 cess results in the elongation of the embryo along the line of the 

 original egg-axis, now the antero-posterior axis — in other words, 

 produces growth in length. 



