ORGANIZERS AND TISSUE DIFFERENTIATION 267 



recipient transplanted tissue, although the latter belongs to a different order. 

 While, therefore, the organismal differentials inherent in the ectoderm of 

 Hyla influnced the type of organ which would develop, the organizer modified 

 the size and cell number in the developing organ. 



We have seen that in early phylogenetic stages the character of the organ- 

 ismal differentials carried by the parts which are joined together is of im- 

 portance in determining the mode of interaction between the different tissues 

 of the two partners. Similarly, we may inquire how a gastrula and a piece of 

 tissue to be grafted into it, one or the other of which is the carrier of an 

 effective organizer, will interact when the transplantations are of a heterog- 

 enous or xenogenous character. 



Some important observations which have a bearing on this problem were 

 made many years ago by Lewis, who found that the optic vesicle was able to 

 induce in embryonal skin the production of cornea, even if skin and optic 

 vesicle belonged to different species. Spemann transplanted ectoderm, repre- 

 senting presumptive abdominal skin, from Triton taeniatus into the anterior, 

 the brain portion of the developing nervous system of Triton cristatus. The 

 latter acted as inductor and transformed the skin into central nervous tissue, 

 which retained, however, the original species characteristics of Triton taenia- 

 tus. Similarly, after heterotransplantation of ectodermal tissues from Triton 

 taeniatus into the gill region of Triton cristatus, the gill which developed from 

 the taeniatus tissue under the influence of cristatus organizers retained the 

 characteristics of the taeniatus species. 



More recently, Mangold showed that the mesodermal structures and chorda 

 of the host could act as organizers even towards heterogenous presumptive 

 ectoderm transplanted into different species of Triton (cristatus, alpestris and 

 taeniatus). The differentiation of the transplant under the influence of the 

 heterogenous organizer tissue proceeded in the same way as after homoio- 

 transplantation ; the rudimentary embryos consisted of constituents of two 

 different species, which, uniting harmoniously, thus represented chimaeras. 

 More extensive investigations of organizer functions following transplanta- 

 tion into different species with varying degrees of relationship were made, 

 especially by Zeinitz, Bytinski-Salz, and Schotte. While in the majority of 

 such experiments tissues from different species were transplanted into Triton 

 taeniatus, Bytinski-Salz carried out also the reciprocal transplantation of parts 

 of Triton taeniatus into a number of nearly or distantly related species. In 

 these experiments either the organizer tissue was transplanted into the bearer 

 of the recipient substratum, or the recipient tissue of a strange organism was 

 transplanted into the carrier of the organizer tissue. If in this way tissues from 

 a more distant species were made to act on each other, difficulties arose irl 

 certain instances, although even after transplantation of anuran tissues into 

 Triton, organizer effects could occasionally be observed. 



Whether under such conditions an organizer effect occurs depends essen- 

 tially upon three sets of factors : ( 1 ) The relationship between host and trans- 

 plant and the influence of organismal differentials; (2) the effect of toxic 

 substances. While to a certain extent the degree of toxicity may be influenced 



