EXPERIMENTAL FISH EMBRYOLOGY 



389 



4. Remove small parts of the nerve cord (stages #17 to #20). This may also be 

 done by suction, but it is difficult to limit the amount removed. Not more than 

 the equivalent of 2 somite lengths should be removed, and a minimum of the ad- 

 jacent notochord and lateral mesoderm. 



5. Remove parts of the brain, also by the suction method. This operation seems to 

 be less drastic for the embryo than some of the others, but it is somewhat more 

 difficult to localize the ablation. No two embryos will be identically treated, 

 hence individual records are required. 



There is a quantitative decrease in regenerative or regulative capacity during early 

 development as revealed by the preceding operations. Nicholas and Oppenheimer, 

 using Fundulus, found no permanent damage up to the 16-cell stage of many of the 

 operated forms. "Up to this point the blastomeres may be regarded as totipotent so 

 long as one-half of the original content of the egg is left untouched. " (Nicholas & 

 Oppenheimer, 1942.) Following this stage there is rapid reduction in regulation, 

 with progressive differentiation. (See Enami 1958, Proc. Jap. Acad. 34:44 & 50 for 

 ablation experiments on caudal neurosecretory organs of fish larvae such as Fun- 

 dulus) 



D. Transections of the Spinal Cord of Motile Stages: (stage #23) 



The nerve cord is transected (without removal of any tissue) with a sharp steel nee- 

 dle or iridectomy scissors, avoiding injury to the notochord if possible. The tran- 

 sections should be at different levels and records should include description of the 

 type of movement upon development to stage #26. Histological analysis will deter- 

 mine the degree of damage and reconstitution. 



DISCUSSION: 



With the increasing physiological complexity of the embryo there is a decrease in the 

 regulative capacity, noted first at the 16-cell stage. When as much as 50% of the proto- 

 plasmic material of the 16-cell stage is removed in Fundulus (Nicholas & Oppenheimer, 

 1942) defects appear and few of the fish gastrulate, but the same proportion of proto- 

 plasmic material can be removed at the previous (8-cell) stage and this will be followed 

 by complete regulation. Following the 16-cell stage there is a rapid drop in regulation, 

 so that at the blastula stage the embryo can tolerate a loss of not more than about 20% of 

 the protoplasmic mass. 



(From Nicholas & Oppenheimer 1942: Jour. Exp. Zool. 90:127) 



