EXPERIMENTAL FISH EMBRYOLOGY 



387 



b. Injury, Ablation and Recovery of Fish Embryos 



Nicholas and Oppenheimer (1942) have found that up to the 16-cell stage the Fundulus 

 blastonneres are totipotent, and that removal of as much as 50% of the embryonic proto- 

 plasm can be survived by the embryo. On the basis of their work, the following proce- 

 dures are given. 



The early stages may be operated on through the chorion, (either by direct pricking with 

 a steel needle or through a window), or after the removal of the chorion. The first pro- 

 cedure is not so easily controlled and pressure factors may be involved, and yet it has 

 the advantage of protection of the embryo against bacteria infection. The second method 

 (i. e. , decapsulating the embryo) involves the possible hazard of infection but removes 

 the tension factor and allows more accurate operational technique. (See previous section 

 for method of decapsulation. ) Both should be attempted. Use any fish egg available, 

 such as Oryzias, Betta, Paradise or Hemichromis. 



ov/^i^ 



Schematic representation of successive 

 stages in the transformation of the blasto- 

 derm to the embryo. The blastoderm (B) 

 slowly expands over the yolk (Y), as is 

 shown in Figs. A, B and C. As gastrula- 

 tion commences (Figs. D and E) the cells 

 are piled up at the periphery of the blasto- 

 derm to form the germ ring (G. R. ) and the 

 embryonic shield (E. S. ); the central por- 

 tion of the blastoderm becomes the extra- 

 embryonic membrane (E.M.). During the 

 course of gastmlation the blastoderm 

 gradually covers the yolk (diagrams F, G, 

 H and I); late in gastrulation a refractile 

 streak (N) visible in the shield represents 

 the keel of the central nervous system. 

 Fig. J shows the extent of embryonic 

 differentiation a few hours after the yolk 

 is completely covered; O. V. , optic 

 vesicle; F B. , forebrain; M.B. , mid- 

 brain; H. B. , hind-brain; N. C. . nerve 

 cord; S. somite; M. unsegmented meso- 

 derm. 



The egg is drawn in profile in all figures - 

 except figure E, which represents the 

 stage shown in Fig. D seen from the 

 animal pole. 



(From Oppenheimer 1936: 

 Jour. Exp. Zool. 73:405) 



Removal or Destruction of a Blastomere : Two to 4-cell stage embryo. 



1. With sharp-pointed steel needle invade the chorion directly over one of the blasto- 

 meres of a 2 or 4-cell stage, and rupture it. Avoid damage to the yolk and con- 

 tiguous blastomeres. 



2. Make a small window in the chorion, directly over the blastomeres. Prepare a 

 micro-pipette with terminal bore slightly smaller than the diameter of a single 

 blastomere. If the edges of the pipette opening are rough, so much the better 



