TRANSPLANTATIONS 261 



THE EYE 



(See also page 253 and 283) 



The eye is a composite organ made up from brain (neural) ectoderm and head ectoderm 

 (lens). Transplantations may be made at various stages, and of constituent parts, but it 

 is the purpose of this exercise merely to determine to what extent the entire optic vesicle 

 and overlying ectoderm can adjust to a new site on the host, a site devoid of the normal 

 second cranial nerve. 



1. Secure embryos (Anura stage #17 or Urodela stage #25) and place them in the oper- 

 ating Syracuse dish over Permoplast and in depressions, in Standard Solution. The 

 optic vesicle protuberance can be easily located on the right side of the head. Pre- 

 pare one embryo as the host by excavating a hole in the ectoderm and underlying 

 mesoderm in the lateral body wall; just anterior to the position of the hind-limb bud; 

 or in the tail bud. 



2. Remove the entire optic vesicle on the right side of the donor, including a good por- 

 tion of the diencephalon. With the overlying ectoderm intact, transfer the entire 

 transplant on a needle to the host site and pack it into place with a hair loop, and hold 

 it for 30 minutes with a piece of cover slip. The eye anlage' is a rather compact unit 

 and since there is less yolk and mesoderm around it than around other anlagen, it 

 may not adhere so readily to the host site. For this reason the excavated site should 

 be somewhat deeper than the thickness of the transplant, and the surrounding ecto- 

 derm should be allowed to partially close over the transplant. 



This procedure can be varied in a number of ways. Extra eyes may be transplanted close 

 to the site of the host eye to determine the degree of fusion or interference; an older 

 anlage' m^y be transplanted to a younger host (the reverse is likely to give negative re- 

 sults); the eye may be so oriented that the optic vesicle faces inward (in which case an 

 extra covering of ectoderm must be provided); and xenoplastic transplants should be at- 

 tempted, particularly between species of Amblystoma. (See Hewitt, 1935, Jour. Exp. 

 Zool. 69:235 for a study of xenoplastic transplants of eye rudiments between various 

 Anura and Amblystoma. ) (Some ingenious transplantations, rotations, and regenerations 

 of older, larval eyes, have been made by Stone and Zaur, 1940. ) (See exercise on Eye 

 Field Operations and on Wolffian Regeneration. ) 



As in all transplant experiments, make drawings or photographs immediately after the 

 operation and at appropriate intervals thereafter. Sectioned material at a later stage 

 will answer the question relative to innervation. 



THE BALANCERS 



(See also page 85-87) 



Balancers are paired, slender, rod-like appendages which project from the side of the 

 head slightly behind and below the level of the eyes. They are found in Triturus and in 

 Amblystoma, (except for A. tigrinum), but never in Rana. They serve as supports which 

 hold the head off the bottom, preventing the larva from losing its balance before the de- 

 velopment of functional forelimbs. They consist of an epithelial membrane whose glands 

 secrete a mucoid substance at the tip and a mesodermal central core with a nerve and 

 bloodvessels (see Bodenstein, 1943). 



