THE ROLES OF THE OPTIC VESICLE AND OTHER HEAD TISSUES IN INDUCTION BY ANTONE G. JACOBSON

DEPARTMENT OF BIOLOGICAL SCIENCES, STANFORD UNIVERSITY Communicated by V. C. Twitty, May 12,1955 The induction of the amphibian lens, although one of the most extensively in- vestigated phenomena in amphibian development, has continued to present per- plexing if not paradoxical features. The early work of Spemann and of Lewis established that the optic vesicle has an active role in lens induction.' In some species, however, it was found that lenses could be formed in the absence of the optic vesicle. While Spemann had found this to be the case in Rana esculenta,' others, notably Woerdeman,2 were unable to obtain independent lenses in esculenta. Studying determination of ciliary polarity in of Amblystoma punctatum, Twitty3 observed that low temperature slows the process of determination less than it retards the rate of morphological development. Following this lead, Ten Cate,4 working in Woerdeman's laboratory, has shown that the difference between Woerde- man's and Spemann's results with esculenta is attributable to the lower tem- perature at which Spemann kept his embryos prior to the extirpation of the retinal anlage. There still remained the problem of identifying the inductors involved in evoking a lens in the absence of the optic vesicle. Liedke5 has demonstrated in A. punctatum that "lens competence is acquired during the open stages through activation by the anterior ento-meso- derm." Considering together the effects of temperature and the role of the "an- terior ento-mesoderm," Twitty6 has suggested that

. probably in all species lens formation is ordinarily elicited by the action of at least two supplementary or mutually reinforcing inductors, the head mesoderm and the retinal rudi- ment. Under average environmental circumstances and in most species the component events of development are geared with one another in such manner that the epidermis is insufficiently differentiated at the stage to be fully receptive to the action of the mesoderm, and requires a subsequent and final impetus from the optic vesicle. At low tem- peratures, however, and especially in species like R. esculenta in which such treatment causes morphological development to lag substantially behind chemical differentiation, the lens epidermis has already become sufficiently mature and responsive at to require no further incitement beyond that provided by the head mesoderm. This suggestion has been substantiated in the present study by combining on a single species, Triturus torosus, the temperature and "head mesoderm" experi- ments and also by the results of isolating from the young neurula presumptive lens epidermis together with the subjacent tissues in vitro. As shown in Figure 1, the temperature at which torosus embryos are reared prior to excision of the retinal anlage in the open stage has a pronounced effect on the percentage of animals which develop lenses in the absence of the optic vesicle. An example of an with an independent lens is shown in Figure 2. Liedke5 had found in A. punctatum that gastrula epidermis transplanted to the lens site develops a lens only if it is in position during neurulation. Lenses de- veloped in the graft if the host was an early neurula but not if the host was a late 522 Downloaded by guest on September 27, 2021 Voi.. 41, 1955 ZOOLOGY: A. G. JACOBSON 523

neurula. In the present experiments with torosus, this same distinction was evident but not so clear cut. Lenses developed in 71 per cent of the grafts to early neurula hosts and in 28 per cent of the grafts to late neurula hosts.

Ce LaJ 70 z -Jl 60 z Li Z 50 0iCL 0 z 40 I 1-

(C) 30 0 m > 20

Li.. 0 F- 10 Li U LLi CL 0 mmm 6mmmmmmJ 6mmmmJ 6mmm. 6. 6... 5 9 13 16 19 25 DEGREES CENTIGRADE FIG. 1.-Percentage of torosus embryos which develop a lens in the absence of the optic vesicle after rearing the groups of embryos at the indicated temperatures prior to excision of the retinal rudiment in the early neurula (stage 16). All groups were kept at 190 C. after the operation.

In the early neurula the presumptive lens epidermis is underlaid directly by the entodermal wall of the archenteron; the anterior edge of the lateral plate meso- derm lies just posterior to the lens site. Presumptive lens epidermis from young neurulae was isolated in combination with (1) the subjacent entodermal wall of the archenteron; (2) the anterior portion of the lateral plate mesoderm; and (3) these two tissues combined (Fig. 3). Lenses were induced in a very substantial percent- age of cases in all three series. No lenses have been found in presumptive lens Downloaded by guest on September 27, 2021 FIG. 2. Transverse section of a 12-mm. torosus larva in which a lens has developed in the absence of the retina. The embryo was reared at 13° C. until the early neurula stage, when the right retinal anlage was excised. FIG. 3. Section of an explant of presumptive lens epidermis combined with the subjacent ento- dermal wall of the archenteron and the anterior portion of the lateral plate mesoderm; taken from a young torosus neurula (stage 16). A lens has been induced from the epidermis. Downloaded by guest on September 27, 2021 Voi,. 41, 1955 ZOOLOGY: A. G. JACOBSON 525

epidermis isolated alone. These explants were taken from embryos which had been reared at 9 ° or at 13 ° C. The results of these experiments indicate that lens induction is the result of the synergistic action of the optic vesicle and of the nonneural tissues underlying the presumptive lens epidermis during neurulation. In fact, these latter tissues may by themselves induce lenses, without the action of the optic vesicle, especially in embryos that have been reared at appropriate temperatures. The emergence of the lens under the influence of more than one inductor finds a close parallel in the in- duction of other head organs, such as the otocyst, the olfactory organ, and the hy- pophysis.7 1 H. Spemann, Embryonic Development and Induction (New Haven: Yale University Press, 1938), chap. iii. 2 M. W. Woerdeman, Koninkl. Ned. Akad. Wetenschap., Proc., 42, 290-292, 1939. 3 V. C. Twitty, J. Exptl. Zool., 50, 319-344, 1928. 4 G. Ten Cate, The Intrinsic Development of Amphibian Embryos (Amsterdam: North-Holland Publishing Co., 1953), chap. vi. 5 K. B. Liedke, J. Exptl. Zool., 117, 573-591, 1951. 6 V. C. Twitty, in Willier, Weiss, and Hamburger, Analysis of Development (Philadelphia & London: W. B. Saunders Co., 1955), pp. 402-414. 7 J. Holtfreter, Growth (suppl.), pp. 117-152, 1951. Downloaded by guest on September 27, 2021