Ill SECONDARY AND MINOR INDUCTIONS 465 



experiment, Grobstein and Dalton (1957) compared the transmission of the inductive 

 activity through three kinds of fiUers: 



a. TA, with pore diameters of 0,8 [i., and a flow rate of 363 ml/min/cm^ 

 h. THA, with pore diameters of 0,45 [jl and a flow rate of 67,2 ml/min/cmz 

 c. TV, with pore diameters of 0,1 [i, and a flow rate of 1 1,5 ml/min/cm^ 



a permits a plain induction of tubules; h, a definite induction, although any direct contact 

 is practically excluded; c, a lesser induction but still clearly positive, while electron-micros- 

 cope exploration fails to show any contact of cytoplasmic expansions. 



There is, however, a new rebound to this multiface question: the formation of 

 definite metanephric tubules has been observed in pieces of the blastem taken 

 at the 1 1 th day before any contact with the Wolffian duct and grafted in the anterior 

 chamber of the eye (Grobstein and Parker, 1958). Would the "milieu interieur", 

 in the case of the aqueous humor of the eye, contain the necessary inductor? An 

 answer will probably not be long to be found to this new puzzle. 



Fig. 97. A tentative explanation of induction phenomena in the case of the salivary epithe- 

 lium (A) and the spinal cord (B) . For the latter organ two different properties are considered, 

 the ability to provoke chondrogenesis (C + ) and to induce the formation of kidney tubules 

 (K). See text. From Grobstein 1955a. 



Finally, Grobstein's interpretation of the two phenomena which he was able 

 to analyse most thoroughly is shown in Fig. 97. For the salivary system (J), three 

 tissues interact: i. the epithelium, whose tendency to form dichotomously branch- 

 ing tubules can be disrupted by direct exposure to a surface; 2. the "condensed" 

 specific or autogenous capsular mesenchyme immediately surrounding the epi- 

 thelium bud, which at least protects and may promote its morphogenetic tendency ; 

 3. the general mesenchyme, which inhibits the morphogenetic tendency of the 

 epithelium, possibly because of an overly strong anti-spreading effect. It is added 

 that the protection exerted by the capsular mesenchyme may depend on a 

 different, more gelated matrix. For the inducing activity of the spinal cord (5), 

 attention is drawn to the glycoprotein matrix which covers the more dorsal region. 



Literature p. 48^ 



