Eye 



409 



cal factoi'S (e.g., the mere mechanical pres- 

 sure of one part on the other) do not seem 

 to be responsible, since Ballard (personal 

 communication) finds that the growth of the 

 A. punctatum cup is not increased by graft- 

 ing a supernumerary lens of the same species 

 into the pupillary space. 



When the entire eye (optic vesicle to- 

 gether with lens epidermis) is grafted be- 

 tween embryos of these two species, it pur- 

 sues its genetic or predestined growth rate 

 independently of the size of the host (Har- 

 rison, '24; Twitty and Schwind, '31). Unlike 

 its component parts, the size increase of the 

 total organ appears to be relatively unaf- 

 fected by the growth intensity of its imme- 

 diate environment. Thus an A. tigrinum eye 

 becomes disproportionately large for its 

 A. punctatum host, and an A. punctatum eye 

 grafted to A. tigrinum preserves its charac- 

 teristic small size in spite of the fact that it 

 is nourished by a much larger and more 

 actively growing host. 



This greater autonomy of the eye as a 

 whole is not surprising, however, since the 

 organ develops as a relatively self-contained 

 and discontinuous entity. This is not true 

 of its constituent parts, the retina and lens, 

 which are very intimately engaged through- 

 out their development. Numerous examples 

 of local size adjustments between parts in- 

 tricately related during their development 

 could be offered, including fiirther instances 

 drawn from the eye itself. Not only in 

 amphibians (Twitty, '32), but in birds (Am- 

 prino, '49, '50) and mammals (Wessely, '20) 

 as well, the extrinsic ocular muscles and also 

 the skeletal structures associated with the 

 eye partly adjust their size to that of the 

 bulb.* Thus when an A. tigrinum vesicle 

 and lens is grafted to an /I. punctatum em- 

 bryo, the eye muscles developing from host 

 tissue become considerably enlarged be- 

 yond their prospective size, in keeping with 

 the more massive proportions of the organ 

 to which they are now attached. It is of 

 interest to note that their enlargement is 

 effected by increase in the number, not size, 

 * In the chick there appears to be a relatively- 

 fixed amount of scleral mesenchyme, whose disposi- 

 tion over the surface of the eyeball and, accordingly, 

 the thickness of the scleral cartilage, are determined 

 by mechanical stress resulting from growth of the 

 bulb. Following experimental reduction of the size 

 of the eye and thus of its surface area, cartilage 

 comes to invest the eyeball as a markedly thickened 

 layer (Weiss and Amprino, '40). In the amphibian 

 the amount of scleral cartilage which forms seems 

 to be more closely proportional to the size of the 

 eye. 



of the constituent muscle fibers (see also 

 Amprino, '49). Since ordinary functional 

 hypertrophy of muscles results trom enlarge- 

 ment of the individual fibers, without in- 

 crease in their number, one is led to suggest 

 that the adjustment in muscle size following 

 heteroplastic transplantation of the eye is 

 attributable to factors more subtle than mere 

 functional demand. 



The influence of the eye on the dimen- 

 sions of the orbit as demonstrated by ex- 

 perimental alteration of bulb size in various 

 animals is discussed by Washburn and Det- 

 wiler ('43) in relation to its bearing on 

 problems of comparative physical an- 

 thropology. 



In addition to grafting embryonic eyes 

 between species of different growth rate, 

 there is another means of producing dis- 

 harmonious size relationships between the 

 organ and the rest of the animal. This is by 

 exchanging eyes between younger and older 

 larvae, whether of the same or different 

 species. Thus when an eye of an advanced 

 larva is replaced by one from a donor half 

 the size or age of the host, the grafted 

 organ is at first much "too small" for its 

 new host. This experiment was performed by 

 Twitty ('30) and Twitty and Elliott ('34), 

 who found that the small eye, through its 

 more active growth, overtakes the normal 

 host organ and thereby restores the normal 

 size equilibrium. Their results indicated 

 that during its period of regvilatory growth 

 the young transplant exceeded its customary 

 rate of increase, and to account for this ac- 

 celeration it was postulated (Twitty, '40) 

 that with increasing age the larval blood 

 stream affords progressively richer nutrient 

 opportunities. This appeared to be confirmed 

 by chemical determinations that showed an 

 increase with age in the concentration of 

 nonprotein and amino acid blood nitrogen 

 (Twitty and van Wagtendonk, '40). Mean- 

 while, however, Handford ('45, '48) has re- 

 ported that young eyes continue to grow at 

 their normal or predestined rate following 

 transplantation to older hosts. Until the is- 

 sues raised by his results can be subjected 

 to further investigation, conclusions con- 

 cerning the control of proportionate eye 

 growth must accordingly be held in reserve. 



REGENERATION OF THE EYE 



Complete regeneration of the urodele eye 

 is possible following removal of all except 

 a relatively small remnant of the organ. 

 Reconstitution is effected mostly through a 



