38 BASIC METHODS FOR EXPERIMENTS 



Eggs and sperm can 51so be obtained by stripping the ani- 

 mals. The stripping should be gently performed by applying 

 pressure on the abdomen toward the anus. The eggs are best 

 fertilized dry, i.e., the eggs and sperm are first mixed and then 

 sea-water is added; this is generally true for teleostan eggs. 

 Personally, I prefer to use normally laid eggs. 



Eggs of Fundulus are extremely hardy and the fish are easily 

 reared in the laboratory. They are therefore excellent for 

 many problems in experimental embryology. However, they 

 do present to the experimenter one serious obstacle; namely, the 

 chorionic membrane. This, fortunately, at least in the later 

 stages of development, can be removed. 



Nicholas and later Armstrong (1928) have used the egg of 

 Fundulus with its chorion removed. Practically, the normality 

 of its development is not thereby impaired. Removal of the 

 membrane is a most useful procedure for experimental work. 

 I give Armstrong's method in detail: 



He removes the chorionic membrane at the stage of closure 

 of the blastopore under a binocular dissecting microscope with 

 dissecting needles and iridectomy scissors. "In removing the 

 membrane special precautions must be taken to avoid injury to 

 the embryo. The following procedure gave uniformly good 

 results: the point of a sharp dissecting needle was pushed into 

 the membrane and the egg rotated so that the tip of the needle 

 within the membrane could be held against the bottom of the 

 dish at an acute angle. A second needle was then drawn across 

 the under side of the first needle so as to make a slit in the 

 membrane large enough for the introduction of the point of the 

 lower blade of the iridectomy scissors. By this means the mem- 

 brane was readily removed, without exerting any pressure on 

 the embryo. The naked embryos were kept over night in sea- 

 water, during which time a few embryos, which had been injured 

 in the removal of the membrane died. The mortality was 

 usually 4 to 5 per cent." 



Literature 



Armstrong, P. B. 1928. The antagonism between acetic acid and 

 the chlorides of sodium, potassium, and calcium as manifested 

 in developing Fundulus embryos. Journ. Gen. Physiol., Vol. 11. 



