140 organisers: inducers of differentiation 



induced to differentiate in particular ways under its influence, and 

 the labile determinations of these host tissues, whatever they may 

 have been, are obliterated^ and overridden. A cell-region which 

 possesses a labile determination to become epidermis may be made 

 to become neural folds. The organiser can, as previously mentioned 

 (p. 46), even override the presumptive distinction between the 

 germ-layers. For instance, a piece of presumptive ectoderm 

 (epidermis) implanted just below the dorsal lip will be carried into 

 the interior of the embryo, and there may give rise to a portion of 

 any of the following organs : vertebral centrum, myotome, lateral 

 plate, pronephros (mesodermal), notochord, or gut- wall (endo- 

 dermal).^ Presumptive neural folds can also form myotomes and 

 pronephros. Similarly, pieces of presumptive mesoderm grafted 

 into the region of presumptive ectoderm will (provided of course 

 that they are taken at the stage prior to chemo-differentiation) form 

 epidermis. The determination of epidermis, however, appears to 

 be less rigorous, and already differentiated epidermis can be made 

 to form conjunctiva (p. 178). 



§2 



It must be remembered that in the production of an end-result, 

 such as a differentiated structure, two sets of factors are involved : 

 first, the causal agent, in this case the organiser ; second the material 

 acted upon, the tissues. Examples of this resultant effect will be 

 given in the following paragraphs. 



The action of the amphibian organiser is not species-specific, 

 i.e. it can induce the formation of axial structures when grafted into 



^ Another example of the overriding of a previous labile determination is 

 provided by the Gephyrean v^^orm Bonellia. This form shows extreme sexual 

 dimorphism, the female being about the size of a plum with a proboscis a yard 

 long, while the male is only a few millimetres in length, and lives parasitically in 

 the uterus of the female. The larvae which hatch from the eggs all pass through 

 an indifferent stage. If such larvae do not come into contact with an adult female, 

 they themselves undergo development into females, by means of processes for 

 which the larva must presumably possess some sort of determination. But this 

 determination can be overridden if the larva comes into contact with an adult 

 female and settles on her proboscis. The proboscis secretes a substance which 

 induces in the larva the development of the male characters, involving reduction 

 of the anterior end of the body, and differentiation of the male reproductive 

 organs (Baltzer, 193 1). 

 ^ Mangold, 1924. 



