174 organisers: inducers of differentiation 



experiments. If the leg imaginal discs of mature blow-fly larvae are 

 cultured in vitro in media of inorganic salts or of larval body-fluid, 

 they will remain healthy for several days, but will not develop. If 

 now the larval body-fluid is replaced by pupal body-fluid, or if the 

 cultures had been put up in this medium straightaway, the leg 

 imaginal discs become evaginated and grow into segmented limbs.^ 

 They do not, however, develop beyond the stage corresponding to 

 the fifth day of pupal life, and this is possibly due to the absence 

 from the culture medium of some substances necessary for further 

 development. At all events, it is clear that the differentiation of the 

 leg-rudiments is dependent on changes which occur in the body- 

 fluid at the onset of pupal life. Other experiments have shown that 

 the process of moulting, so characteristic of insect development, is 

 a reaction of the epidermis to a substance in the body-fluid, 

 amounting to a hormone.^ 



Next, there are the effects of other substances carried in the 

 blood stream. The classical example of this concerns the pig- 

 mentary pattern of the late embryo of the fish Fundulus. The 

 pattern is due to the pigment-cells arranging themselves along 

 the blood-vessels of the yolk-sac, i.e. in situations where the 

 maximum amount of oxygen is available.^ 



Then, there are the trophic effects of nerves, such as the de- 

 pendence of the differentiation of taste-buds in the fish Amiurus 

 upon contact with the nerve endings of the facial nerve* (see p. 430). 

 Finally, there are the moulding effects of pressure and tension upon 

 the form, size, and intimate structure of such organs as sinews and 

 blood-vessels (see p. 432). 



With regard to differentiation of the type seen in the prefunc- 

 tional period of development, there are, at the opposite extreme 

 from the organiser phenomena, effects primarily mechanical in 

 nature. An example of these is seen in the dependence of the arms 

 of the pluteus larvae of sea-urchins upon the growth of the larval 

 skeleton. In the absence of the skeleton, no arms are produced : if 

 an abnormal number of skeletal spicules are formed, a correspond- 

 ing number of arms are produced : if the spicules are abnormal in 



^ Frew, 1928. 2 von Buddenbrock, 1930; Bodenstein, 1933. 



^ J. Loeb, 1912. 



* Olmsted, 1920; G. H. Parker, 1932 A, b. 



