EXPERIMENTAL MORPHOGENESIS 81 



basis of explanation for some results, obtained from the 

 youngest germs of some other animal species, which other- 

 wise would seem to be rather irreconcilable with what our 

 Echinus teaches us. 



You know, from the second lecture, what a gastrula of 

 our sea-urchin is. If you bisect this gastrula, when it is 

 completely formed, or still better, if you bisect the gastrula 

 of the starfish, either along the axis or at right angles 

 to it, you get complete little organisms developed from the 

 parts : the ectoderm is formed in the typical manner in the 

 parts, and so is the endoderm ; everything is proportionate 

 and only smaller than in the normal case. So we have at 

 once the important results, that, as in the blastula, so in the 

 ectoderm and in the endoderm of our Echinus or of the 

 starfish, the prospective potencies are the same for every 

 single element : both in the ectoderm and in the endoderm the 

 prospective value of each cell is a " function of its position ' 



(Kg- 9). 



But a further experiment has been made on our gastrula. 



If at the moment when the material of the future intestine 

 is most distinctly marked in the blastoderm, but not yet 

 grown into a tube, if at this moment the upper half of the 

 larva is separated from the lower by an equatorial section, 

 you will get a complete larva only from that part which 

 bears the " Anlage " of the endoderm, while the other half 

 will proceed in morphogenesis very well but will form only 

 ectodermal organs. By another sort of experiment, which 

 we cannot fully explain here, it has been shown that the 

 endoderm if isolated is also only able to form such organs 

 as are normally derived from it. 



And so we may summarise both our last results by 



6 



