organisers: inducers of differentiation 193 



Each of the various inductive effects accordingly covers a wide 

 area, or field, and the intensity of the induction decreases with 

 increasing distance from a sub-central point in each field (see 

 Chap, vii, p. 223). 



If, now, w^e stop to inquire which structures are responsible for 

 the inductive effects, the answer appears to be in most cases that 

 each field is dependent, not on one but on several other structures. 

 The organiser for the neural tube induction in the trunk region 

 appears to be the segmented mesoderm; this, which of course is 

 derived from the invaginated organiser, is known to induce neural 

 tube when grafted beneath strange epidermis. Here, the converse 

 experiment has been performed, and strange epidermis has been 

 grafted over the derivative of the organiser. The inductive action 

 which produces portions of brain, etc., in the head, appears to 

 proceed from the neural crest, which is also capable of inducing 

 cartilage, ganglion-cells, sense-organs, and ear-vesicles.^ At the 

 same time, the induction of ear-vesicles can be performed homoio- 

 genetically, by ear-vesicles, just as fore-limb and pronephros can 

 induce fore-limb and pronephros respectively. 



The formation of a tail is the combined and coordinated 

 result of a number of inductive influences. The elongation and 

 stretching of the notochord and musculature, and the metameric 

 arrangement of the latter, are dependent on the presence of 

 neural crest mesenchyme; dorsal and ventral fins are formed 

 when neural tube is present ; in the absence of neural crest 

 mesenchyme, the initial elongation of the tail-bud stops, and 

 regression sets in. 



We see, in general, that as a result of the inductive capacities of 

 the organiser and of certain other structures (themselves the result 

 of induction by the organiser), the amphibian embryo at the neurula 

 stage is already what may figuratively be called a physiological 

 mosaic of formative stimuli, leading to the demarcation of fields, 

 each of which represents the sphere of action of a particular type of 

 inductive effect. We shall see in the next chapter that these fields 

 constitute one of the most important features of the next or mosaic 

 stage of development. 



^ In these experiments the grafts do not appear to have come into close contact 

 with the brain itself of the host-embryo. The homoiogenetic inductive capacity 

 of the brain has, however, been established by other work (see above, p. 147). 

 HEE 13 



