84 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



Our knowledge of the origin of organs has been greatly- 

 added to by the researches of Spemann, Mangold, and 

 others on the development of Amphibians. Before 

 gastrulation it is possible to exchange for a fragment of 

 ordinary epidermis a fragment of that region of the epi- 

 dermis which would normally be folded in to form the 

 nerve-cord ; these fragments are found to be still 

 " plastic," and the " presumptive " nerve-cord becomes 

 ordinary skin, and the presumptive skin normal nerve- 

 cord ; such transplants can even be made from one species 

 to another. After gastrulation, however, although no 

 visible differentiation has taken place, the plasticity has 

 been lost and the exchange can no longer be made success- 

 fully ; the fate of these regions of epidermis has been 

 settled and sealed, presumably by some chemical alteration 

 of the tissues. 



The " axial structures " on the back of the embryo, i.e. 

 the nerve-cord, notochord, and somites, arise in the meridian 

 of the dorsal lip of the blastopore as it travels backwards 

 towards the vegetative pole. If, before this time, the 

 upper part of the blastula is " scalped " off and replaced 

 at right angles to its original position, the axial structures 

 maintain their relation to the meridian of the dorsal lip, 

 although the cells from which they are formed have 

 been rotated. The position of the axial structures is 

 determined by the dorsal lip of the blastopore, which, 

 as we have seen, arises from the grey crescent of the egg. 



Still more remarkable, it is found that if the dorsal lip 

 from one embryo is grafted into the flank of another, it 

 will there induce the formation of an extra and imwanted 

 set of axial organs, which would otherwise never have 

 arisen ; these do not arise from the cells grafted-in, but 

 from the cells of the host embryo under the influence 

 of the implant. It is not even necessary that the implant 

 should come from an embryo of the same species or genus 

 as the host. 



The dorsal lip of the blastopore, which provokes the 

 formation of a set of axial organs in normal or abnormal 

 situations, is called the " organiser." The evidence 

 suggests that from it chemical substances diffuse out into 

 the surrounding tissues. The organiser cannot make its 



