THE TOPOGENESIS OF THE EMBRYO 101 



parallel to the germ surface, whereas in another area they 

 tend to extend at right angles to it. In yet another area they 

 may assume a club-shape by enlargement of the part pointing 

 towards the centre of the germ, and narrowing of the other 

 end. This will cause the cell to move into the interior, and 

 thereby give rise to an invagination of the superficial material. 

 It has indeed been proved experimentally that different parts 

 of the early gastrula of amphibians show tendencies for differ- 

 ent movements. Part of the marginal zone tissue of one gastrula 

 can be grafted into the ventral side, or 

 into the animal field, of another early 

 gastrula. A blastopore will then be 

 formed by invagination in the graft, all 

 of which is rolled into the interior, often 

 together with a number of cells of the 

 host. This shows that marginal zone 



material must have the potency to roll „^ ^-r , , ^ 



... ' -li. 1 ^ J- ^ Fig. 36. "Hyperblastu- 



m actively, simultaneously extending to ^^,r ^^ Triton formed 



form an archenteron. On the other hand, ^y the animal half of 



the material of the animal field shows an egg. Strong ex- 



a tendency to expand parallel to the pansion of the aninial 



n o /ir\n-i\ J. J. 1-1 J. material leads to fold- 



surface. Spemann (1931) cut twoblastu- ^ After Ruud 



lae in halves along the equator, and 



grafted the two animal halves together. This resulted in a 

 germ with an excessively expanded ectoderm, forming many 

 loose folds and flaps (Fig. 36). This can be explained as follows. 

 In normal development, the expanding ectoderm takes the place 

 of the marginal zone material that is rolled in during gastrula- 

 tion. But, in the germ resulting from the fusion of two animal 

 halves, no rolling in takes place, and there is therefore no room 

 for the expanding ectoderm, which is obliged to fold. The 

 material of the vegetative field shows hardly any capacity for 

 active movement. It seems to be more or less passive in gastrul- 

 ation; only the very first beginnings of the blastopore furrow 

 in this area appear to be due to an active power of invagination 

 of the cells, which assume a club shape, as mentioned above 

 (Vogt; 1922, 1929). 

 This power of active movement, or topogenetic potency of 



