EMBRYONIC INDUCTORS AND ORGANIZERS 493 



of specificity dependent on developmental stage and differing in different 

 species and at different body-levels may be concerned in reaction of epi- 

 dermis by lens formation to an optic cup or other living or dead inductor 

 is not known, and practically nothing is known concerning intensity or 

 degree of inducing power of optic primordia or of the effects of operative 

 procedures, transplantation, explantation, etc., upon it. 



ORIGIN OF PATTERN IN THE LENS 



The lens possesses a polarity normally coincident in direction with the 

 optic axis. Mangold (1931a, pp. 272-74) regards the optic cup as deter- 

 mining this polarity but holds that in lens developing independently of 

 the optic cup the polarity is already determined in the lens primordium. 

 Spemann (1936, p. 56) believes that the polarity is determined by the 

 inducing action from one side. Since the lens in early stages is essentially 

 similar to a bud, its polarity may result from this form of development 

 rather than from the induction itself. The central region of the bud, pre- 

 sumably the most active region in early stages, becomes the lens proper, 

 the peripheral regions forming the capsule. Instead of becoming an elon- 

 gated axis like most buds, direction of growth of the originally central 

 region appears to undergo reversal in direction and to be directed toward 

 the cavity of the lens primordium and the epidermis, whether because of 

 pressure on the retina or some other factor (Fig. 162). In any case it is 

 evident that a polarity may originate in relation to the differential activity 

 represented in the differential growth of the lens primordium and quite 

 independently of an inductor. The lens primordium buds from the epi- 

 dermis, and the lens proper buds into the interior of the lens vesicle formed 

 by separation of the bud from the epidermis, but the axis of the lens is 

 in the same direction as that of the original bud, though direction of 

 growth is reversed. The only relation of an inductor to the lens polarity 

 may be that of initiating a region of activity grading off from a center. 

 Alterations of lens polarity under experimental conditions offer no diffi- 

 culties to this conception. Experiments by Dragomirow (1929) show al- 

 teration in relation to other developing parts than an optic cup. Early 

 optic cups with presumptive lens epidermis were so transplanted that the 

 epidermis was between the developing otic primordium and the optic 

 cup. Lenses developed with two axes at differing angles to each other, 

 sometimes opposed, one in the usual relation to the optic cup, the other 

 vertical to the surface of the otic vesicle. Here the effect of the otic ves- 



