PATTERNS OF DEVELOPMENTAL PROGRESS 719 



suggested that the experiments indicate a similar organismic environment 

 regulating growth rate in the two species, rather than fixity of the intrinsic 

 rate. With this possibility in mind, Twitty made heteroplastic transplan- 

 tations of optic primordia, using individuals of the same two species but 

 of different age or growth stage and found that growth of the eye, rela- 

 tive to that of the host, could be retarded or accelerated. With older 

 A. pundatum as host, younger eyes of A. tigrinum grow very rapidly, 

 while growth of host eye and normal eye is prevented by light feeding. 

 With younger A . tigrinum as host, growth of transplanted eye is inhibited, 

 even with liberal feeding and rapid growth of host. "Expression of the 

 growth capacity of the eye is primarily a function of the relation between 

 the physiological conditions in the organ and in the environment provided 

 by the host, rather than of mere growth in size of the latter" (Twitty, 



1930). 



When the optic vesicle alone, without lens or corneal ectoderm, is 

 transplanted from one species to the other, the developing lens apparently 

 influences growth of the transplanted eye. The optic vesicle of A . tigrinum 

 transplanted to A . pundatum grows more rapidly than the eye of the 

 host species but more slowly than an A. tigrinum transplant of vesicle 

 with lens ectoderm. This retardation is attributed to the host lens, which 

 is "too small" for the transplanted optic cup. Conversely, the A. punc- 

 tatum vesicle without lens ectoderm, transplanted to A . tigrinum, grows 

 more slowly than that of the normal host but is accelerated by the 

 A . tigrinum lens, which is "too large." Transplants of lens ectoderm alone 

 from A . tigrinum to A . pundatum give rise to lenses too large for the eye. 

 Their growth is retarded, but growth of the host optic bulb is increased. 

 In reciprocal transplants of lens ectoderm the lenses are too small, but 

 their growth is accelerated and that of the bulb retarded.^ Evidently the 

 species-specific growth rates of these parts of the eye, which normally 

 develop in a definite relation to each other, may be mutually altered when 

 the parts originate from different species. This seems not to be true for 

 all parts of the embryo, for heteroplastic transplants of portions of the 

 shoulder girdle between the same two species do not influence growth of 

 host parts but may themselves be altered in form. On the other hand, 

 transplants of the larger limb primordium of A . tigrinum to A . pundatum 



^ Harrison, 1929a, b; Rotmann, 1939, in experiments with other species. It has also been 

 shown that heteroplastic eye transplants between the same two species influence growth of 

 various parts of the brain, of the cartilaginous capsule developing about the eye, and of the 

 eye muscles (Twitty, 1932). 



