43() ^- H. Krecker, 



possible tliat the ectoderm clianges its character independently of the 

 iieoblasts and tliat the neoblasts simjjly gatlier about the point at 

 which tliis change occurs. In all early stages examined it was found 

 that the neoblasts appeared about the woiind before the ectoderm 

 underwent a transformation, but, of course, that fact does not of ne- 

 cessity disprove that the ectoderm changes its character independently 

 of the neoblasts. However, the exception cited in which no change 

 in the character of the ectoderm cells occured in the absence of the 

 neoblasts is of considerable significance and from the evidence at his 

 command the writer is inclined to think that the neoblasts have some 

 influence in bringing about the changed character of the ectoderm 

 cells in their vicinity. There is no direct evidence as to how this is 

 accomplished but presumably there is a chemical Stimulus. 



The metamorphosis of the ectoderm cells is evidently part of the 

 process of redifferentiation or rejuvenation which must be undergone 

 before these cells can form new structures. If this change in the cha- 

 racter of the ectoderm cells is due to the neoblasts it iraplies that the 

 neoblasts have a redifferentiating effect upon the cells of the ectoderm. 

 On first thought this may seem rather improbable but nevertheless 

 there is some evidence to support such a view in spite of the some- 

 what scanty scientific data concerning the redifferentiating influence 

 of one part of the organism upon other parts. The results obtained 

 by Spemann and others in transplantation experiments concerning 

 the correlation between the optic cup and lens formation in Amphibia 

 show that an organ may have a reditferentiating effect upon other 

 parts of the body. Of course in these instances the tissues concerned 

 had not reached the adult stage of differentiation. However, in an 

 adult organism certain structures exert what is presumably a chemical 

 influence upon other structures, and, indeed, upon the entire body 

 as for example the influence of the thyroid glands. It is well known 

 that the removal of these glands causes serious disturbances in the 

 organism and that the placing of a portion of a thyroid gland beneath 

 the skin or merely the injection of thyroid extract will restore normal 

 conditions. An instance of substance from an embryo affecting adult 

 structures is shown in the renewed activity of the mammary glands 

 of an adult rabbit due to the injection of an extract from a foetal rabbit. 

 In view of these facts a redifferentiating influence of the neoblasts 

 upon the ectoderm cells is not so improbable as it may at first appear. 



