THE ORIGIN OF NEW CELLS AND TISSUES 203 



prevented, the animal dies. Nussbaum showed how the turning back 

 takes place in an animal while it remains on the bristle. The everted 

 foot-end begins first to turn back, pushing into the central cavity. 

 When it comes to the bristle it passes to one side of it, and continuing 

 to turn back the foot passes out of the mouth, drawing the rest of the 

 body after it. 1 The last act of the turning can take place only by 

 tearing away through one or both sides, and this is often done. The 

 bristle may still remain sticking to the body through one side, or even 

 remain through both sides if the body has, after tearing through, 

 healed up around the bristle. The process of turning back may take 

 place quite quickly, and had been overlooked by Trembley, who 

 trusted too confidently to the presence of the bristle sticking through 

 the animal. 



The method by which the turning back of the layers takes place 

 was not, it appears, clearly described by Nussbaum in his first paper, 

 for his account seems to imply, in certain passages, that the ectoderm 

 may slide over the endoderm during the process, rather than that 

 both layers always turn together. Ischikawa, who studied the problem 

 later, gave a clearer account of the method of turning back. Nuss- 

 baum has stated^ in a later paper that he had described essentially 

 the same process. 



In conclusion, it can be definitely stated that a transformation of 

 ectoderm into endoderm cannot take place in hydra. Ischikawa also 

 tried removing the endoderm from a piece by spreading it out and 

 then killing the inner layer by weak acid applied with a brush, but 

 pieces of this sort failed to regenerate a new endoderm. 



Tower has recently stated that if a living hydra is put into a 

 strong light from an arc lamp of 52 volt 12 ampere capacity, that 

 is focussed on the animal (after passing through an alum cell), the 

 ectoderm cells fly off, but if the animal is kept, it subsequently pro- 

 duces a new ectoderm. Whether all the ectoderm is lost, or only the 

 larger neuro-muscular cells, was not made out. 



One of the most unexpected discoveries of recent times in con- 

 nection with the problem of regeneration is the renewal of the 

 extirpated eye of triton and salamandra. Colucci first discovered 

 in 1891 that if the eye is partially removed a new eye develops from 

 the piece that remains and that the neiv lens develops from tJie margin 

 of the bulb. Wolff, a few years later, not knowing of Colucci's 

 results, also found that after extirpation of the lens of triton, by 

 making an incision in the cornea, a new lens develops from the edge 

 of the old iris. Wolff pointed out the great theoretical importance 

 of this result. The experiment has been repeated and confirmed by 



1 The foot sometimes pushes out through one of the slits made by the bristle instead of 

 out of the mouth. 



