284 



EYE FIELD OPERATIONS 



In the Anura the eye lorming n:iaterials have been localised in the late blastula as lying 

 about 40 to 50° above the equator (see section on "Vital Staining and Morphogenetic 

 Movements"). In the early neurula when the boundaries of the medullary plate are barely 



discernible, the eye anlage' occupies a circular region with a 

 diameter of about 1/3 the greatest breadth of the neural plate, 

 at its antero-lateral boundary (see diagrams). During the 

 elevation of the neural folds there- is a ventro-lateral evagin- 

 ation of the newly formed brain cavity to form the optic ves- 

 icles. A narrow strip of median material separates the two 

 eye anlage. This median strip forms the chiasma and a por- 

 tion of the lamina terminalis. Actually, therefore, the two 

 eye anlagen are not completely separated. 



t!\ OPTIC veacLES 



DORSAL vew OF EMBRYONIC 

 FOReeRAX 



(Redrawn from Spemann, 1938) 



Mangold (1928) found that when the presumptive eye field had 

 been underlain with archenteric roof, as early as the medium 

 sized yolk-plug stage (Anura stage #11 or Urodele stage #15), 

 the field had eye forming potencies when introduced into a 

 younger blastocoele. Mangold (1931) discussed the possible 

 relation of the whole organism, the anlage' itself, and the 

 immediate environment of the anlage in the segregation of the eye forming potencies. It 

 is one of the purposes of this exercise that the student determine the exact time and ex- 

 tent of the determination and segregation of the eye-forming potencies. There are, how- 

 ever, species differences. (See Glossary for definitions of "double assurance" in rela- 

 tion to Rana esculenta and dependent differentiation. ) 



Exploratory dissections: 



1. Using Anura stages #16 to #18 (or Urodela stages #20 to #30), dissect away 

 the ectoderm of the head region to expose the optic vesicles and the central 

 nervous system. This may be done with living material, or with formalin- 

 hardened specimens. 



2. Anesthetize Anuran stages #21 and older (or corresponding Urodele embryos) 

 in 1/3, 000 MS 222 (or 0. 04% chloretone) and dissect out the entire optico- 

 ocular apparatus. Note the po- 

 sition and state of development 



of the lens. The larger tadpoles 

 should be pinned down to a Per- 

 moplast base and covered with 

 lens paper (except for the head). 



3. Remove the lens of later stages as 

 follows. Pierce the skin in front 

 of and ventral to the eye with a 

 sharp glass needle, push the nee- 

 dle backward or upward between 

 the eye and the cornea parallel to 

 the cornea. Pierce again at the 

 posterior or upper margin of the 

 eye. Cut the cornea by inserting 

 the needle beneath it and rubbing 

 a scalpel against it. Take two 

 small but unequal sized hair loops 

 and pass them over the lens, from 

 opposite sides, and then pull them 

 apart. In this manner the lens 

 will be cleanly removed (as with 

 scissors) with minimum of damage 

 to the other parts of the eye. 



GLiSS HICRO-NEE&LC 



METHOD OF CUTTING THE CORNEA TO RE- 

 MOVE THE LENS FROM THE AMPHIBL\N EYE 

 TO TEST FOR WOLFFL\N (LENS) REGENERATION 



(Needle is inserted through cornea on one side of the 

 iris, passed between the cornea and the side of the 

 iris. The needle is lifted against the cornea, and a 

 sharp scalpel is then scraped against the needle, pro- 

 viding a cutting edge to cut a slit in the cornea. ) 



