organisers: inducers of differentiation 173 



The intersexual males demonstrate that the determination of the 

 scales must in them occur as the resuh of the streaming out of some 

 chemical agent, responsible for initiating male-type determination, 

 from the body over the wings along the course of the veins. There 

 exists what Goldschmidt calls a " stream of determination ". Slight 

 variation in the resistance of the various veins will lead to large 

 individual variation in the precise course of the flow\ Normally 

 after a time the flow reaches every part of the wing. But in the 

 intersexes, if the switch-over from male to female metabolism 

 occurs during the time occupied by the outstreaming of this sub- 

 stance, all the parts which it has not yet reached will develop as 

 female, and the male-determined areas, later becoming coloured 

 with melanin, remain as a record of the early course of the flow. 



We may presume that there is some passage of a determining 

 substance from the organiser to other regions in the pre-gastrulation 

 stage of the amphibian egg (see p. 139); but this is the only case 

 known where one must postulate a flow of such a substance along 

 anatomically diflFerentiated channels. Much remains to be cleared 

 up as regards this phenomenon. For instance, it manifests itself in 

 certain rare cases among female intersexes, but the wings of these 

 are usually whole-coloured and of male (dark) type. 



§7 

 Organiser phenomena are clearly special cases of what Roux 

 termed dependent differentiation. As noted in Chap, in, p. 54, we 

 will use the term in its restricted sense for cases in which the diflFer- 

 entiation of one part depends, in one way or another, upon the 

 presence and previous differentiation of another part. The factors 

 concerning dependent differentiation fall into several rather difTerent 

 categories. 



It will be convenient first to give brief consideration to those 

 effects which commonly begin to operate after the functional 

 period of development has started. These are of various distinct 

 types. First, there are the morphological effects of hormones, such 

 as the influence of the gonad hormones upon secondary sexual 

 characters in vertebrates. Another example, this time from inver- 

 tebrates, concerns the differentiating capacities of rudiments of 

 insect organs, which have been tested by means of explantation 



