14 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



of the frog that, even though the frog takes up water from the pond 

 quite readily through its skin, there may not be enough time for the 

 water to pass into the frog, then into the blood circulating in the walls 

 of the oviduct, and thence into the eggs. The forces involved in the 

 imbibition of water by proteins are very large, and there is no doubt 

 that, if considerable water did gain access to the eggs, the results would 

 be as disastrous as Nussbaum found. 



This rather compHcated story of the formation of the jelly envelope 

 may, then, be summarized as follows. The viteUi pass into the oviduct. 

 There they form the nuclei round which collects the insoluble product 

 of a clotting system, in which a factor resembling the thromboplastin 

 of blood contributes to a transformation that has something in common 

 with the similar transformation that occurs when blood clots. The 

 protein is not the same, and the rest of the process probably differs 

 considerably. This protein layer is fairly concentrated. When it has 

 formed, the eggs pass into the ovisac, where they become distributed 

 in a fairly concentrated solution of a different protein. When they are 

 ejected by the frog, the mass passes into water, salt is removed and the 

 protein precipitates, forming the second and outer layer of jelly. Both 

 layers now imbibe water, and in twenty-four hours or so achieve 

 maximum size. In this process, the outer layer is stretched by the 

 swelling of the imier layer, and gives way except at the points of 

 contact of the spheres, where there is no stress. The fmal result is that 

 a mass of adherent spheres is formed, with channels in between. The 

 fmal percentage of structural protein in a blood-clot and in frog-jelly 

 is about the same, but the clot and the jelly approach this concentration 

 from opposite directions: the jelly swells, but the blood-clot is formed 

 full size at once and, indeed, tends to shrink. 



Ecological Properties of the Jelly 



In some species, for example Rana sylvatica or R. aurora aurora, the 

 envelope contains algae, as described by Dickerson (1906) and by 

 Wright and Wright (1949) respectively, and Dickerson suggested that 

 the algae may provide oxygen for the embryos. Occasionally, the 

 eggs of R. temporaria are green, although this seems usually to be due 

 to algae on the outside, but it is easy to see that, if the outer layer of 

 jelly, which is a precipitate, is formed in water green v^th algae, then 

 algae will be enclosed when the jelly forms. It is interesting that in the 

 American species the algae are so constantly present that they are 



