EMBRYONIC INDUCTORS AND ORGANIZERS 495 



ing chemical agents, in his experiments chiefly to butyric acid and acetone. 

 The free lenses were assumed to be induced by fragments of the optic 

 primordia scattered through the head region in consequence of blastolysis; 

 and the hypothesis that the vertebrate lens is induced in all cases was 

 advanced, but conclusive evidence supporting it is lacking. Lenses also 

 appear in normal relation to the optic cups of teratological forms when 

 these reach the epidermis, but experiments demonstrating induction seem 

 to be lacking. That the normally lentogenous teleost epidermis can de- 

 velop lens in absence of optic cup is evident from some of these teratologi- 

 cal forms, and there is little doubt that the supernumerary lenses develop 

 from other epidermis. The teratological forms concerned are differential 

 modifications of development. It may be suggested that in those experi- 

 mentally produced the differentially inhibiting action of the agent de- 

 creases or perhaps abolishes the differential in the head region. With the 

 activation associated with recovery following return to water, the inte- 

 grating and ordering factors in the head region being less effective than 

 normally and different regions being more alike, local activations in the 

 epidermis at certain stages, either induced by underlying parts or per- 

 haps originating in the epidermis, may bring about formation of several 

 lenses independent of eyes. The appearance of supernumerary adventi- 

 tious polarities in Corymorpha with essentially similar treatment may be 

 recalled (pp. 416-17). 



Data bearing on lens induction in the chick are few, but it has been 

 shown that in stages from primitive streak to four somites the optic cup 

 can induce lens in ectoderm of head, neck, or trunk, in later stages only 

 in head and neck. Up to five lenses or lentoids may develop in relation to 

 one transplant (Alexander, 1937). Earlier experiments also show or indi- 

 cate induction in the normally lentogenous or adjoining epidermis.-"^ 



OTHER INDUCTIONS IN VERTEBRATE DEVELOPMENT 



As development progresses, other inductions of organ systems and 

 organs, most extensively studied in amphibians, take place in particular 

 parts of the body or under experimental conditions at certain develop- 

 mental stages. Some of these have been mentioned in connection with the 

 field concept (chap, viii), and certain others are briefly considered here. 



RETINA AND CONJUNCTIVA 



Although retinal and tapetal parts of the optic primordium become 

 visibly different in early stages (Fig. 162, A), the tapetum may still give 

 ■tt DanchakofE, 1924, 1926; Reverberi, 1929; Willier and Rawles, 1931; Rawles, 1936. 



