224 The Nature of Biological Diversity 



sources of information on the developmental role of microenviron- 

 ment — embryonic induction and the reaggregation of dissociated cells. 

 The information which I shall summarize raises the interesting possi- 

 bility that there are important relationships among the mechanisms 

 of aggregation, induction, and cytodifferentiative control. 



Embryonic induction long has been recognized as relevant to cyto- 

 differentiative control because it involves developmental dependency 

 — the differentiative course of one group of cells in the embryo is 

 different depending upon whether or not it comes into association with 

 a second group. The possibility of microenvironmental effects is raised 

 because the association between the two tissues normally is extremely 

 close. 



In lens induction by optic vesicle, for example, the morphological 

 association is so close, as judged by histological sections and by efforts 

 to separate the two mechanically, that mechanisms have been pro- 

 posed (Weiss, 1958) which assume direct surface-to-surface contact 

 of the cells involved. These assumptions were supported by the finding 

 that interposition of such materials as cellophane between the lento- 

 genic epithelium and the optic vesicle completely blocks the induction 

 (McKeehan, 1951). In recent years, however, evidence has been ac- 

 cumulating that actual contact is not required in certain inductive 

 systems, providing the interspace between has suitable properties. It 

 appears, nonetheless, that even under these circumstances there is a 

 limit to the separation distance across which the reaction can proceed. 

 I should like, first, to recall some of the evidence which seems to 

 justify these statements. 



Let me begin by introducing the experimental system which has so 

 far been most useful in our laboratory. It involves the rudiment of 

 the definitive kidney or metanephros of the mouse. In the 11-day 

 embryo the rudiment consists of two components, the outer meta- 

 nephrogenic mesenchyme and the inner ureteric bud. It is important 

 to recall that the ureteric bud branches to form the collecting ducts 

 of the kidney, while the outer mesenchyme contributes the glomeruli 

 and secretory tubules. If the intact rudiment is placed under suitable 

 conditions in vitro, each component undergoes continued morpho- 

 genesis in sufficiently normal fashion to be recognized in the develop- 

 ing culture and in subsequent histological sections (Figs. 1 to 6). If 

 the rudiment is first treated with trypsin, however, and the ureteric 

 bud removed, the metanephrogenic mesenchyme alone entirely fails 

 to undergo kidney-type morphogenesis under the same culture condi- 

 tions. If the separated mesenchyme is recombined in culture with 

 ureteric bud, or with the dorsal region of embryonic spinal cord, 



