NATURE OF AGENTS WHICH FORM MEMBRANES 453 



of eggs by a dilution of one in a hundred thousand after an exposure of 

 from 30 to 60 minutes; these membranes are formed in the solution 

 itself without transference to normal sea-water. If, however, the 

 action of the saponin solution upon the egg be permitted to continue, 

 membrane-formation is rapidly succeeded by cytolysis and the egg 

 disintegrates. This is illustrated in the preceding figure (Fig. 29), 

 showing the successive effects of a saponin solution (8 drops of J per 

 cent, saponin to five c.c. of sea-water) upon the unfertilized eggs of 

 Strongylocentrotus. It is perfectly clear from these results that the 

 formation of the fertilization membrane represents an initial stage of 

 cytolysis. If, however, the eggs be removed from the saponin solution 

 before manifest cytolysis has occurred, washed in sea-water a number 

 of times to remove the last traces of saponin, and then treated with 

 hypertonic sea-water, normal development occurs and a large propor- 

 tion of the eggs develop into larvae. Precisely the same results are 

 obtained with Bile-salts. 



There remains, however, another class of cytolytic agent which has 

 yet to be considered in this connection, namely the tissue-fluids of 

 unrelated species of animals. The Blood of the mammalia contains 

 cytolytic substances which hemolyze and destroy foreign corpuscles 

 and cells, but not those of the same species. This cytolytic power of 

 blood and other tissue-fluids is greatly enhanced by previous immuni- 

 zation with the foreign cells, but within the body at least it is exercised 

 without previous immunization. It was found by Loeb that this class 

 of cytolytic agents is also capable of causing membrane-formation 

 in the sea-urchin egg. Not every unrelated species of animal, however, 

 would furnish a tissue-fluid or extract capable of causing membrane- 

 formation, in fact only a limited number of forms were found to do so. 

 Loeb considers that this is due to the variable permeability of the eggs 

 for different lysins, and the permeability of the eggs for a particular 

 lysin also seems to vary somewhat in the eggs of different females. 



The first cytolytic agent of this kind to be discovered was that 

 contained in the blood of certain marine worms, namely Dendrostoma, 

 which calls forth membrane-formation in the sea-urchin egg even if it is 

 diluted a thousand times or more with sea-water. Later investigations 

 have shown that a number of other invertebrates yield tissue-fluids 

 or extracts which will cause membrane-formation. Most interesting 

 results were, however, obtained by Loeb with the blood of mammalia 

 or birds. 



The blood-sera of mammals (oxen, sheep or pigs) which have been 

 rendered isotonic with sea-water by the addition of sodium chloride 

 will induce membrane-formation in sea-urchin eggs, but not invariably 

 nor in all of the eggs derived from different females. Eggs from one and 

 the same female will form membranes in some samples of blood and not 

 in others, and again, one and the same sample of blood will form 

 membranes in the eggs of some females but not in others. As in the 

 case of the saponins the membranes are formed in the solution itself, 



