ORGANIZERS AND TISSUE DIFFERENTIATION 265 



epidermis, may be transformed into mesoderm if it is transplanted into a 

 place in the embryo where normally mesodermal structures develop. After 

 further transplantation, these mesodermal tissues, if they are brought into 

 contact with other ectoderm, are able to induce the formation of a medullary 

 plate, and the medullary plate can induce the formation of medullary plate 

 and other nerve structures from ectoderm. 



As stated above, in addition to organizer actions induced from the outside, 

 there are active processes inherent in the tissues themselves, leading to self- 

 differentiation ; during normal embryonal development these two processes 

 seem to cooperate in various combinations, in which the relative importance 

 of each factor may differ quantitatively. Various kinds of interaction may 

 thus be produced experimentally. We have referred previously to an instance 

 in which an organ, although much differentiated, still retains its ability to act 

 as an organizer. The optic disc in certain stages of embryonal development 

 can induce lens formation in some species only in the cephalic ectoderm, while 

 in other species at a certain stage of differentiation, also ectoderm of the rest 

 of the body can be made to develop into lens. Now, Mangold has found that 

 the eye-forming substances are determined in the ectoderm a short time after 

 the mesodermal tissues and the chorda, constituting the roof of the archen- 

 teron, have formed and have been able to act on the ectoderm ; it is therefore 

 possible that this contact induces the ability of the overlying ectoderm to 

 differentiate into an eye. However that may be, it can be shown that, from 

 a certain period in embryonal development on, there is manifested in the 

 overlying neural plate, as a result of increasing self-differentiation, an inherent 

 tendency to produce optic vesicles independently of any organizer action. Yet 

 even then, according to Adelman, an organizer action may be associated in 

 its effects with this process of self-differentiation. The roof of the archen- 

 teron of Amblystoma not only tends to reinforce the inherent tendency of the 

 ectoderm to form eyes, but it also modifies the place in the neural plate where 

 the eyes develop. Inherently the median portion of the neural plate has a 

 greater tendency to form eyes by self-differentiation than the lateral parts; 

 but the underlying roof of the archenteron acquires a marked bilateral polarity 

 in the course of embryonal development, and this condition influences the 

 organizer action of this tissue ; the lateral parts of the underlying tissue now 

 gain a greater tendency to induce or to intensify eye formation than its median 

 part, the organizer action dominating over the forces inherent in the neural 

 plates and causing the production of lateral eyes. 



These interferences between self-differentiation tendencies and organizer 

 action can be shown in still another way. If the lateral parts of the neural 

 plates are transplanted together with the underlying organizer tissues, more 

 eyes are formed than would develop without the latter. If, on the other hand, 

 the median parts of the neural plate are transplanted without the underlying 

 tissues, they form eyes just as well ; however, median parts of the underlying 

 tissue, when transplanted with the median neural plate, frequently cause the 

 separation of the eye-forming material into two eyes, while without this 

 tissue, more often only one eye forms. In this case the organizer exerts effects 



