406 



Special Vertebrate Organogenesis 



or explanted vesicles will induce lenses from 

 foreign epidermis (Alexander, '37; van Deth, 

 '40; McKeehan, '51). In other groups of 

 vertebrates the evidence is mostly fragmen- 

 tary or indirect (see Mangold, '31). 



The results of efforts to ascertain the na- 

 ture of the lens-inducing stimulus are of 

 uncertain implication, and can be properly 

 discussed and evaluated only in conjunction 

 with those of similar studies on the primary 

 organizer. Optic vesicles killed by heat or 

 other treatments, although they may in- 

 duce neural complexes including eye vesi- 

 cles, which in turn evoke lens formation 

 from the host ectoderm, seldom if ever in- 

 duce lenses directly (see Needham, '42). 

 The formation of isolated lenses has been 

 elicited, however, by a variety of foreign 

 tissues, such as fresh salamander liver and 

 boiled salamander heart (Holtfreter, '34). 

 One is also reminded of the isolated lenses 

 and lentoids appearing in Fundulus embryos 

 following treatment with acetone and alco- 

 hol, as reported by Werber (who erroneously 

 attributed his results to the inductive action 

 of optic fragments or substances dispersed to 

 other parts of the embryo through the 

 "blastolytic" effects of the toxic agents). 

 Toivonen ('49) finds that alcohol-treated 

 liver, which he has shown to be relatively 

 specific for the induction of head structures, 

 may occasionally stimulate the formation 

 of independent lenses. This archencephalic 

 inductor is thermostable, soluble in organic 

 solvents, and, in fractionations, accompanies 

 the fatty acids and nucleoproteins. 



Although these experiments point to the 

 intervention of specific diffusiWe substances 

 in lens induction, interpretation and extra- 

 polation of the results must be undertaken 

 with caution. One is reminded, for example, 

 of Holtfreter's ('34, '45, '47) finding that 

 many instances of neural induction by ar- 

 tificial agents appear to be the indirect 

 consequence of toxic action by the foreign 

 tissues and chemical substances employed. 

 Even unfavorable pH, by producing sub- 

 lethal "shock," can seemingly launch chemi- 

 cal sequences in gastrula ectoderm cells that 

 lead to their neuralization; and it is con- 

 ceivable that the foreign tissues or tissue 

 extracts that elicit lens formation are equally 

 nonspecific in their mode of action. 



Even more difficult than identifying the 

 agents responsible for lens induction is the 

 task of analyzing the manner in which their 

 effects upon the lens epidermis are mediated 

 and expressed. Although little concrete in- 

 formation is available on these points, the 



problem has been approached in a novel 

 and stimulating way by Weiss ('50). Weiss 

 has long stressed the importance of physical 

 factors in development, and has recently 

 presented a concept of induction based on 

 the consequences, at the molecular level, of 

 simple contactual relationships between 

 cells. It has often been demonstrated that 

 embryonic inductors are effective only when 

 they are in direct physical contact with the 

 reacting tissue, and this is well exemplified 

 in the case of the eye; interposition between 

 optic vesicle and epidermis of a narrow gap 

 or an extremely thin layer of mesenchyme 

 cells is sufficient to block lens induction. 

 Weiss's concept, as applied to the eye, is 

 that lens formation results from selective 

 accumulation of particular species of mole- 

 cules at the surfaces of the epidermal cells, 

 in response to specific physical affinities 

 that exist between these molecules and those 

 arrayed superficially in the cells of the 

 vesicle. His concept "illustrates how inter- 

 faces, by selectively adsorbing, orienting, 

 and aligning molecular key species from 

 the cellular content, produce the physical 

 conditions of spatial order which enable 

 stereochemically complementary compounds, 

 such as antibody-antigen or enzyme-sub- 

 strate systems, to interact most effectively. 

 According to this concept, . . . when a cell 

 comes into contact with another body whose 

 surface is settled with molecules with spe- 

 cifically formed end groups, . . . molecules 

 of conforming configuration will be trapped 

 by their correspondingly shaped counter- 

 parts. Thus, a specific fraction of the cell 

 content will gradually become concen- 

 trated and segregated along the contact 

 surface [and] ... a whole train of 

 events determining the future chemical his- 

 tory of the cell has thus been set into mo- 

 tion." 



Weiss's student, McKeehan ('51), finds 

 that at the time of contact between optic 

 vesicle and ectoderm in the chick, the cells 

 of the ectoderm become elongated and ori- 

 ented in the direction of the subjacent 

 retinal cells. This response, together with the 

 firm adhesion developing during this period 

 between retina and lens ectoderm, is pre- 

 cisely the type of behavior to be expected 

 in terms of Weiss's thesis. An important fea- 

 ture of this concept is that it accounts for 

 the induction of a lens or other structure 

 without invoking the actual passage of dif- 

 fusible substances from inductor to reacting 

 tissue. 



Once the lens placode is established, its 



