102 



THE TOPOGENESIS OF THE EMBRYO 



Fig. 37. (a-b) normal gastrulation, seen a: from the vegetative side, 

 and b: fro'm the left side; (c-d) exogastrulation. The arrows indicate 

 the directions in which the material moves. After Vogt and 



Holtfreter. 



the cells, is very clearly shown if the normal co-operation of 

 the migratory tendencies in gastrulation is disturbed in some 

 way or other. Holtfreter (1933) obtained such a disturbance 

 in newt eggs by taking them out of their capsules at an early 

 stage, and transferring them into certain salt solutions. The 

 resulting disturbance of gastrulation is known as "exogastrul- 

 ation". No invagination of the archenteron, accompanied by 

 rolling in of the material of marginal zone and vegetative field 

 takes place. On the contrary, an evagination occurs at the edge of 

 the blastopore, because marginal zone material moves through 

 the blastopore furrow to overlie the vegetative field. The result is 

 a germ in which endoderm and chordamesoderm are not sur- 

 rounded by, but lie altogether outside the ectoderm. The latter 

 forms an empty, folded pouch which may eventually become 

 completely separated from the rest of the germ at the blasto- 

 pore edge (Fig. 37). Now it is a remarkable fact that all the 

 other movements of gastrulation, and also part of the sub- 

 sequent movements of neurulation, take place in the normal 



