408 



Special Vertebrate Organogenesis 



larger. We know that it is first determined 

 as retinal material while it is still a portion 

 of the open medullary plate, and hence 

 before it actually evaginates to form a vesi- 

 cle. Thus if a piece of ectoderm from some 

 other region, say ectoderm ordinarily des- 

 tined to form flank skin, is grafted into this 

 region at a sufficiently early stage it will 

 subsequently form the eye vesicle. One of 

 the possibilities considered by Rotmann is 

 that the area of retinal material thus deter- 

 mined, and hence the initial size of the eye 

 vesicle, is arbitrarily prescribed by the in- 

 ductor itself. An alternative is that the 

 inductor merely releases retina-forming po- 

 tentialities, and that the actual scale of the 

 rudiment is an independent expression of 

 genetic qualities in the reacting tissue. This 

 was tested by grafting gastrula ectoderm of 

 T. cristatus in place of the future eye region 

 of the smaller T. taeniatus embryo. The eye 

 vesicle later forming from the graft was 

 generally of the same size as the normal 

 host eye, suggesting that the inductor not 

 only "determines" the retinal material as 

 such, but also specifies its scale. In a few 

 cases, however, the vesicle was larger than 

 that of its host; and in the reciprocal ex- 

 periment, T. taeniatus ectoderm on the larger 

 T. cristatus formed a vesicle appropriate in 

 size for the donor species, that is, much 

 smaller than the normal host vesicle. 



Incidentally, initial conformity of the op- 

 tic vesicle to the size prescribed by the host, 

 as in the first combination, does not mean 

 that its subsequent growth rate and eventual 

 size are similarly altered. Rotmann's experi- 

 ment was performed independently by the 

 writer (unpublished), and it was found that 

 such vesicles, althovigh starting at the same 

 size, later grew into eyes considerably larger 

 than the normal organs of the host. This is 

 in accordance with the more rapid larval 

 growth rate of the donor species. 



Rotmann ('39) obtained a more definite 

 answer to the question of initial size in 

 similar experiments with the lens. Here the 

 scale of the rudiment is apparently an ex- 

 pression of genetic qualities in the lens- 

 forming tissue itself. Thus T. cristatus ecto- 

 derm grafted over an optic vesicle of T. 

 taeniatus forms a lens that already adheres 

 to the T. cristatus scale at its earliest recog- 

 nizable stage of development. This lens was 

 formed at the behest of the small T. taeni- 

 atus eye vesicle, but apparently the "instruc- 

 tions" do not inclvide specifications concern- 

 ing the size of the structure elicited. By 

 using haploid embryos as donors of the 



grafted ectoderm, Rotmann later ('40) 

 showed that the initial size of the rudiment 

 results from the mobilization, not of a spe- 

 cific number of epidermal cells, but of an 

 epidermal area of specific extent. The in- 

 duced haploid rudiments, at the stage when 

 they covild first be clearly demarcated, con- 

 tained about 70 per cent more cells, but 

 were of the same total dimensions, as diploid 

 lenses of the same species. This is in keep- 

 ing with the recent finding of McKeehan 

 ('51) that the area of epidermis incorporated 

 into the lens rudiment in the chick coincides 

 precisely with that enjoying direct physical 

 contact with the optic vesicle. McKeehan's 

 observation does not in turn, however, seem 

 to harmonize well with Rotmann's earlier 

 report that lenses induced from T. cristatus 

 ectoderm by the small T. taeniatus vesicles 

 are of T. cristatus scale from the beginning. 



These results on the heteroplastic trans- 

 plantation of the lens raise a further ques- 

 tion that is of even greater pertinence to 

 the problem of eye growth. A chimeric eye 

 is created in which the lens and optic 

 cup are of disharmoniovis proportions, and 

 one may inquire what the consequences will 

 be during subsequent growth of such an 

 organ. This question has also been studied 

 by Rotmann, but it was first tested by Harri- 

 son ('29) in experiments with two species 

 of Amblystoma differing markedly in rate 

 of growth. The eye of the more rapidly 

 growing species, A. tigrinum, is much larger 

 than that of A. punctatum, so that when the 

 optic vesicle of one and the lens epidermis 

 of the other are combined into a single eye, 

 a potential maladjustment of considerable 

 magnitude is created. This does not ma- 

 terialize, however. Optic cup and lens exert 

 a mutual influence on the growth of one 

 another, with the result that the chimeric 

 eye eventually develops harmonious pro- 

 portions. For example, an A. punctatum cup, 

 when combined with an A. tigrinum lens, 

 exceeds its normal size, while in turn the 

 lens fails to attain its usual dimensions. 

 These results have been confirmed and ex- 

 tended by Ballard ('39). 



No direct information is available concern- 

 ing the way in which this growth adjustment 

 is effected, although one is tempted to sug- 

 gest that differences in metabolic rate be- 

 tween the two components of the chimeric 

 eyes may somehow be involved. Wills ('36) 

 and Hopkins and Handford ('43) have 

 shown that the level of oxygen consumption 

 is decidedly higher in the more rapidly 

 growing species, A. tigrinum. Simple physi- 



