TRANSPLANTING EXPERIMENTS 85 



influence felt across a cut where there is no contact ; and 

 it is possible to graft indifferent tissue into contact with an 

 oFganiser and infect it with organising properties. 



Similar transplanting operations have given much in- 

 formation about the differentiation of various organs. For 

 instance, the rudiment of the eye may be transplanted into 

 such unlikely situations as the wall of the abdomen, and 

 will there differentiate by itself into a typical optic cup. 

 On the other hand, the development of the lens of the eye 

 is determined by the presence of an optic cup behind the 

 ectoderm from which the lens arises. Thus if the 

 rudiment of the cup is transplanted, no lens arises in the 

 " proper " place on the head whence the primordium has 

 been removed, but a lens does form from the ectoderm 

 covering the optic cup in its new situation in the abdomen. 

 There may be curious differences in these respects between 

 closely related species. Thus in one frog, Rana esculenta, 

 ordinary epithelium will not form a lens ; but the optic 

 cup of this species can provoke lens-formation from the 

 ordinary epithelium of another species, such as R. fusca, 

 in which lens-formation is dependent on the optic cup as 

 described above. 



At a later stage in differentiation the function of the 

 organs begins to play a part in differentiation. Thus the 

 length of the intestine in tadpoles may be influenced by the 

 diet. Again, in early stages the veins and arteries are 

 alike, and it is possible to transplant a section of vein into 

 the course of an artery, where eventually it becomes thick- 

 walled and elastic under the influence of the higher blood- 

 pressure ; whereas veins, where the blood-pressure is 

 lower, are ordinarily thin-walled and flaccid. 



Generalisations. — (i) The ovum theory or cell theory. — 

 All many-celled animals, produced by sexual reproduction, 

 begin at the beginning again. " The Metazoa begin where 

 the Protozoa leave off " — as single cells. Fertilisation does 

 not make the egg-cell double ; there is only a more com- 

 plex and more vital nucleus than before. All development 

 takes place by the division of this fertilised egg-cell and its 

 descendant cells. 



(2) The gastrcea theory. — As a two-layered gastrula stage 

 occurs, though sometimes disguised by the presence of 



