290 



EYE FIELD OPERATIONS 



OPTIC VESICLES 



In the Anura the eye forming materials have been localised in the late blastula as 

 lying about 1+0° to 50° above the equator (see section on "Vital Staining and Morphogene- 

 tic Movements"). In the early neunila when the boundaries of the medullary plate are bare- 

 ly discernible, the eye anlage occupies a circular region with a diameter of about l/j the 

 greatest breadth of the neural plate, at its antero-lateral boundary (see diagrajns). Dur- 

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

 evaglnation of the newly formed brain cavity to form the optic 

 -FOREBRAH vesicles. A narrow strip of median material separates the two 

 /^f\ eye anlage. This median strip forms the chiasma and a portion 



""■"" of the lamina terminalis. Actually, therefore, the two eye 

 anlagen are not completely separated. 



Mangold (1S;28) found that when the presumptive eye field 

 had been underlain with archenterlc 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 (1951) discussed the possible re- 

 lation of the whole organism, the anlage itself, and the immedi- 

 ate environment of the anlage in the segregation of the eye form- 

 ing potencies. It is one of the purposes of this exercise that 

 the student determine the exact time and extent of the deter- 

 mination and segregation of the eye-forming potencies. There 

 are, however, species differences. (See Glossary for definitions 

 of "double assurance" in relation to Bana esculenta and dependent differentiation.) 



(Redrawn from 

 Spemann, 1958) 



BEEOLE POIMT 



Exploratory dissections : 



1. Using Anura stages #l6 to #18 (or Urodela stages #20 to #50), dissect away the 

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

 vous 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 

 1/5,000 MS 222 (or 0.0^^ chloretone) and dissect out the entire optico-ocular 

 apparatus. Note the position and state of development of the lens. The larger 

 tadpoles should be pinned down to a Permoplast base and covered with lens paper 

 (except for the head). 



5 • Eemove 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 needle backward or up- 

 ward 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. 

 U. In a somewhat similar 



GUSS HICRO-MEEOLE 



METHOD OF CUTTING THE CORNEA TO REMOVE THE 

 LENS FROM THE AAIPHIBIAN EYE TO TEST FOR 

 WOLFFIAN (LENS) REGENERATION 



(Needle is inserted tlirough cornea on one side 



of the iris, passed between the cornea and the 



lens to emerge tlirough the cornea on the far 



side of the Iris. The needle Is lifted against 



the cornea, and a sharp scalpel is then scraped 



against the needle, providing a cutting edge to 



, ,, cut a silt In the cornea.) 

 remove the entire eye ball 



of adult frogs or salamanders. Such hemorrhage as occurs is generally not of 



any serious consequence (see diagram of adult aye). 



manner, but with scissors. 



