42 EARLY AMPHIBIAN DEVELOPMENT 



By a simple operation, the constituent processes of gastrulation 

 can be dissociated from one another. Removal of a portion of tissue 

 at the animal pole of a blastula leads to closure of the wound by 

 approximation of the cut edges. This results in raising the marginal 

 zone above the equator of the egg. Nevertheless, this zone soon 

 shows its characteristic stretching movements, and decreases its 

 diameter. Normally, of course, this decrease in diameter corre- 

 sponds to the curvature of the egg from the equator to the vegetative 

 pole. But as the marginal zone is now above the level of the equator, 

 it cannot simply grow down over the vegetative hemisphere: in- 

 stead, it constricts the embryo into the form of an hour-glass. 



Meanwhile, an invagination appears in the lower half of the hour- 

 glass, at a place which the marginal zone would normally have 

 reached, but which it has been prevented from reaching by the 

 conditions of the experiment.^ That the process of rolling in or 

 diving beneath the surface is an autonomous one is shown by the 

 fact that isolated portions of the dorsal lip region, when grafted 

 into strange situations in another embryo, promptly proceed to 

 transfer themselves into the interior by this means. 



The fact that all these processes should begin and take place 

 more actively at the dorsal meridian before extending to lateral 

 meridians and eventually all round the egg, is a consequence of the 

 gradient of activity from dorsal to ventral side, mentioned above. 

 While the marginal zone is stretching, overgrowing the vegetative 

 hemisphere, and being invaginated and tucked in round the lip of 

 the blastopore, and the presumptive neural fold region is being 

 stretched and displaced, thus taking the place of the presumptive 

 primitive gut-roof which is being invaginated, the presumptive 

 epidermis region expands and extends by growth so as to cover the 

 area vacated by the presumptive neural folds. This growth-tendency 

 on the part of presumptive epidermis is also shown by isolated 

 pieces when grafted,^ and by two ventral half-gastrulae grafted 

 together : the epidermis of each half tries in vain to overgrow the 

 other (fig. 13), with the result that it is thrown into numerous folds. ^ 



The harmonious co-operation of all these processes, which norm- 

 ally result in gastrulation, can be thrown out of gear by interference 

 with the gradients, and alteration of the relative rates of activity in 



^ Vogt, 1922. ^ Mangold, 1924. ^ Spemann, 193 1. 



