84 GROWTH AND MORPHOGENESIS 



due to the diflfusion of a substance of low molecular weight. Either 

 direct contact or passage of macromolecules is thus required for 

 successful induction. 



Similar results have been obtained and similar conclusions have 

 been drawn by McKeehan (1951) and by De Vicentiis(1952), who 

 worked on the induction of the lens by the eye cup. Again, in- 

 sertion of a cellophane membrane between inductor and reactor 

 completely stops lens differentiation. 



Since membranes with larger pores which would allow free pas- 

 sage of macromolecules also stop lens formation, according to De 

 Vincentiis (1952), it must be admitted that in this case direct con- 

 tact between inductor and reactor, in the sense which Weiss (1947) 

 suggests, is the essential factor. The situation remains more obscure 

 in the case of primary induction. The large pore membranes used 

 for this purpose by Brachet and Hugon de Scoeux (1949) often pro- 

 duced spontaneous neuralization of the axolotl ectoderm, probably 

 provoking a precytolytic condition . No conclusion can thus be drawn 

 from these experiments, which should be repeated on eggs whose 

 ectoblast is less sensitive to injury than are those of the axolotl. 



But, as shown by the work of Grobstein (1955, 1956), direct con- 

 tact between inducing and reacting cells is not always required for 

 induction. Working on the induction of tubules in metanephrogenic 

 mesenchyme, he found that the inducing stimulus is not stopped by 

 the interposition of a "milHpore" membrane. Such a membrane has 

 large pores compared with those of a cellophane membrane but its 

 pores are not large enough to allow the passage of free cells, al- 

 though they can become filled with long pseudopodia which, 

 apparently, never come in direct contact (Grobstein, 1955, 1956; 

 Grobstein and Dalton, 1957). The active substance, which cannot 

 cross a cellophane membrane, can act at a distance of more than 

 80 [ji (Grobstein, 1958). For all these reasons Grobstein (1955, 1956) 

 believes that induction is not brought about through direct con- 

 tact or diffusion of a small molecular weight substance, but through 

 the matrix uniting the cells. Such a conclusion is probably also valid 

 for induction of cartilage by spinal cord, which remains possible in 

 the presence of a millipore filter (Lash and Holtzer, 1958). We see 



