118 



GASTRULATION 



Rotation of the amphibian egg in the gravitational field during gastrulation. 

 (After V. Hamburger and B. Mayer, unpublished. Redrawn from Spemann: 

 "Embryonic Development and Induction," New Haven, Yale University Press. ) 



persists and until the cells have attained a position where their potential 

 energy is lowered to a least possible value. This movement is, however, con- 

 ditioned by the cellular plasticity which is restricted. Thanks to the tensile 

 strength of the cell wall, the attenuated neck portion is subjected to an in- 

 creasing mechanical stress. The surface yields and is pulled inside in the 

 form of the archenteron. 



Schechtman (1942) calls involution an "insinking" and then ex- 

 plains what follows, after having demonstrated that various areas of 

 the pre-gastrula have various types of autonomous movement. He 

 says: 



Gastrulation begins with autonomous movements; the in-sinking of the 

 presumptive pharyngeal endoderm, the stretching of the marginal zone to- 

 ward the blastoporal groove, the forward migration of the internally situ- 

 ated marginal material along the underside of the animal hemisphere. As 

 the stretching presumptive chordal region comes to the edge of the blasto- 

 poral lip, it is progressively carried under by the invagination and involu- 



