iQo organisers: inducers of differentiation 



ventral region of the cup. It has been found that the orientation 

 of the fibres of the lens is also dependent on the eye-cup^, and, in 

 particular, on the position of the choroid fissure. This is proved by 

 experiments on embryos of Rana escidenta at the early neurula stage, 

 in which the presumptive lens epidermis is rotated through 90° : the 

 lens-fibres are nevertheless normally oriented. On the other hand, 

 if the eye-rudiment is rotated so that the choroid fissure comes 

 to occupy an abnormal position, the lens-fibres are also abnormally 

 oriented.'^ Therefore, while the lens is normally self-differentiating 

 as regards its general formation in Rana escidenta at this stage, 

 the determination of the orientation of its fibres is still dependent 

 on the eye-cup. At later stages, rotation of the lens-area of epidermis 

 shows that this orientation becomes self-differentiating also. 



Lastly, it may be noticed from those cases in which a lens can be 

 induced by an eye-cup of a different species, that the lens-inducing 

 capacity of the eye-cup, like the organising capacity of the am- 

 phibian dorsal lip, the avian primitive streak, the hypostome of 

 Hydra, the head of Planaria, and the capsule-inducing capacity of 

 the amphibian ear-vesicle, is not species-specific.^ In this respect, 

 the action of organisers and inducing structures has much in 

 common with that of hormones. Many if not all hormones are 

 similar or identical in widely separated groups: thyroxin from a 

 mammal will metamorphose amphibian larvae; testis hormones 

 from a bull will cause the comb of capons to grow ; adrenalin from 

 a fish will excite vaso-constrictor effects in man. On the other hand, 

 the precise effect produced depends on the reacting tissues, just as 

 it does with the organiser effects during development. The tail and 

 limbs of Anuran larvae react to thyroxin, while those of Urodela do 

 not; the larval epidermis of most Amphibia reacts to thyroxin, while 

 that of the adult never does. The relation of organisers to induced 

 organs, as of hormones to reacting tissues, is thus much less specific 

 than the interaction of hereditary outfits (genomes) from different 

 species, where a difference of generic degree is usually more than 

 sufficient to prevent co-operation. 



^ Dragomirow, 1930. " Woerdeman, 1932. 



^ Woerdeman (1933 b) finds marked changes in glycogen content in the 

 eye-rudiment before and during the period of lens-differentiation. The precise 

 meaning of these, as of similar changes in the organiser region (p. 154), remains a 

 subject for future investigation. 



