264 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



transplanted into a defect in the region of the presumptive medullary plate, 

 is able to develop into parts of the central nervous system and it may give 

 rise to the formation of an eye. On the other hand, if a differentiated medul- 

 lary plate, together with adjoining mesodermal tissue, which will give rise 

 to chorda formation, is transplanted into a gastrula, it may induce in the 

 overlying ectoderm the formation of a medullary plate. We see, then, that 

 an organizer may induce in the recipient tissue either structures of the same 

 kind as the organized tissue, a process we may designate as "isoinduction," 

 or it may induce structures of another kind, "alloinduction," as when archen- 

 teron induces formation of medullary plates. Similar differences in organizer 

 action we mentioned in our analysis of the factors which are potent in trans- 

 plantation in lower adult vertebrates. 



The isoinduction which we mentioned, may be of a very specific character. 

 As Mangold has shown, the medullary plate of the neurula may be divided 

 into four parts in the direction from head to tail, and each part is then 

 found to induce in the host the formation of those organs into which that 

 particular segment of the medulla itself would have developed, although to 

 some extent the effects of the different segments are overlapping. However, 

 the various parts of the underlying mesodermal tissue may also exert corre- 

 sponding specific formative effects, cephalic parts of the roof of the archenteron 

 tending to induce the nervous structures characteristic of the head, the 

 posterior portions tending to induce the tail parts. 



While in some cases the ability of a tissue to act as an organizer may be 

 retained with further development, or may become specifically localized in 

 certain portions of the organizer area, in other cases this power is lost. Thus 

 not only the medullary plate, but also the fully developed brain tissue may 

 function as an organizer for the transformation of ectoderm into medullary 

 plate. Similarly, the embryonal optic disc, as stated above, can call forth in 

 the overlying ectoderm the formation of lens tissue. With further differentia- 

 tion of the optic disc, such action has apparently been transferred to the fully 

 differentiated retina of the Triton eye, which now has gained the power to 

 induce in the iris of the eye the formation of a lens. On the other hand, under 

 certain conditions, with increasing differentiation a diminution or a specific 

 limitation in the capacity to serve as an organizer may be noted. From the 

 dorsal lip of the Triton gastrula, which, as we have seen, acts as a very 

 effective organizer, there develops chorda as well as mesodermal structures; 

 but Mangold has demonstrated that it is only the chorda which preserves for 

 some time the ability to induce the production of a medullary plate from the 

 presumptive epidermis, while the mesodermal structures have lost this func- 

 tion. 



During embryonal development we may have to deal with chain reactions 

 induced by successive organizers. Certain mesodermal structures may induce 

 the formation of the optic disc, and the optic disc in contact with ectoderm 

 induces the formation of a lens ; but here the chain reaction ends, the lens not 

 being able further to act as an organizer. Another chain reaction is the fol- 

 lowing: ectoderm, which under normal conditions would differentiate into 



