ORGAN DEVELOPMENT IN VERTEBRATES 269 



an important part. Even in the absence of the duct, accumulations of what 

 one may consider as 'pre-mesonephric' cells may put in a transitory 

 appearance. Further, the inductive influence of the duct is only effective 

 if it operates on intermediate mesoderm (v^hich has probably been brought 

 to a state of competence by its position in the medio-lateral gradient) ; and 

 this mesoderm can, in the chick at least (Griinwald 1942, 1943), react suc- 

 cessfully to abnormal inductors, such as transplanted pieces of neural tissue. 



Griinv^ald also showed that an inductive reaction is concerned in the 

 production of the metanephros of the chick. Here the inductor is the 

 diverticulum which pushes out from the region where the proncphric 

 duct joins the cloaca. He discovered the remarkable fact that if a piece of 

 the main (mesonephric) region of the duct is substituted for this diverti- 

 culum, and allowed to act on the presumptive metanephric tissue, it 

 succeeds in inducing kidney, but mesonephros rather than metanephros. 

 This is one of the comparatively few cases in which the character of an 

 induced organ is determined by the nature of the inducer rather than by 

 the competence of the reactant. 



Grobstein (195 3 rt, h) is analysing the inductive reaction between the 

 duct diverticulum (uretic bud) and the metanephric mesoderm in the 

 mouse, which is probably rather similar. He fmds that the mesoderm can 

 be induced to form tubules by a variety of different types of tissue, all of 

 which are epithelial in character and to that extent at least similar to the 

 uretic bud which is the normal inducer. An important result is that the 

 inducing agent given off by embryonic spinal cord, for example, can pass 

 through a 20[jl thickness of an artificial porous membrane; this is one of 

 the most direct proofs that induction may be carried out by diffusible 

 chemical substances. Grobstein has used the same methods for investisat- 

 ing other examples of the development of glands which contain both 

 epithehal and mesenchymal components, particularly the submandibular 

 (salivary) gland. By careful exposure to trypsin solutions, he can separate 

 the epithelial and mesenchymal tissues; the epithelium can then be 

 cultured in combination with mesenchyme from its own type of gland 

 or with that from some other organ. He fmds that the epithelium differ- 

 entiates typically only when combined with its own type of mesenchyme; 

 other mesenchymal tissues may partially inhibit the spreading tendency 

 which is usually seen in isolated epithelium, but do not induce the forma- 

 tion of normal tubules. If the epithehum and mesenchyme are separated 

 by a fme-grain fdter membrane, the inhibitory action of foreign mesen- 

 chyme passes through, and the specific effect of like mesenchyme also 

 does so to some extent, though not completely; it induces the formation 

 of tubules but not quite typical ones (Fig. 12.9). 



