Developmental Genetics 



267 



FIGURE 30-4. Effect of injecting 

 pituitary gland extracts into dwarf 

 mice. {See text for explanation.) 



\ n 



44 49 54 



AGE IN DAYS 



64 



presumptive, that is, future, notochordal 

 tissue. When normal presumptive notochord 

 is present, the mesoderm surrounding it is 

 induced to form cartilage and vertebral seg- 

 ments. It would seem reasonable to at- 

 tribute the failure of cartilage and vertebrae 

 formation m T T individuals to the failure of 

 presumptive notochord (which has lost the 

 ability to develop into notochord) to induce 

 the differentiation of mesoderm. Is this, in 

 fact, the nature of the change, '\n T T indi- 

 viduals, in the inductive relationship between 

 these two adjacent tissues? This question 

 can be studied using tissue cultures. A con- 

 trol experiment shows that it is possible to 

 explant presumptive notochordal tissue, from 

 a normal individual, which has wrapped 

 around it mesodermal tissue, from the same 

 or another normal individual, and to obtain 

 the development of cartilage and vertebral 

 segments. Surprisingly enough, however, the 



mesoderm from normal embryos will develop 

 into cartilage and vertebral segments when 

 surrounding presumptive notochord from 

 young T T embryos. Moreover, mesoderm 

 from T r embryos does not form cartilage or 

 vertebrae when surrounding presumptive 

 notochord from normal embryos. We must 

 conclude, then, contrary to expectation, that 

 in the case of T T the normal inductive rela- 

 tionship has been disturbed in an unexpected 

 way, the mesoderm being no longer able to 

 respond to the normal inductive stimuH of 

 presumptive notochord. 



We have already seen how genetic change 

 may influence or direct development of multi- 

 cellular organisms by means of modifying 

 (1) the relative growth rates of parts (Creeper 

 Hmbs) and (2) the over-all growth rate (pitui- 

 tary dwarfism). We have also found that 

 genetic changes can act upon differentiation 

 at a distance by means of (1) a general change 



