THE EGGS AND YOUNG TADPOLES II 



doubt that some of the properties of a part of the jelly o[ the egg de- 

 pend on analogous reactions. 



When the blood is circulating normally in the vessels, it does not 

 clot. As soon as the blood escapes and comes into contact with a surface 

 such as damaged tissues or a glass vessel, an extremely complicated 

 series of reactions begins, in the course of which various factors appear 

 and play their brief part. Among these is thromboplastin. Tliis factor, 

 in the presence of the calcium ions always present, acts on a protein, 

 prothrombin, to produce thrombin, an enzyme. The plasma also 

 contains a soluble protein, fibrinogen, present to the extent of about 

 0-3 per cent. Thrombin converts fibrinogen to fibrin, wliich is 

 insoluble and exists in the form of threads. These threads ramify 

 through the blood and form the skeleton of the clot, in the interstices 

 of v^hich the rest of the blood is entangled. The "problem" that is 

 solved by these reactions is this: the blood is required to remain fluid 

 indefinitely while it is in the vessels, but must become sohd, with as 

 Httle expenditure of material as possible, as soon as it is shed. The 

 "problem" that the frog has to solve is this: the jelly envelope must 

 originate as a hquid, for only a hquid can be secreted by the tubular 

 glands of the oviduct. It must become sohd with as little expenditure 

 of material as possible. When it was reahzed that the percentage of 

 protein in both the structural part of a blood clot and in the jelly of 

 the frog's egg is the same, and that both contain a clotting factor, it 

 became obvious that the parallehsms were too close to be merely a 

 coincidence. 



When the frog ovulates, the vitelh leave the ovaries and pass down 

 the oviducts to the dilated portions of these organs known as the 

 ovisacs. There they wait until the moment of oviposition, wliich takes 

 only a few seconds. The mass of eggs, measuring about 20 to 30 ml, 

 passes through the narrow cloacal opening in this time as if it were a 

 liquid stream. A few seconds later, the eggs become sohd and swell, 

 eventually becoming a mass some three hundred times the bulk of the 

 ovisac eggs. 



Dr. Barnett used eggs laid naturally. Since he also reported the 

 existence of a similar clotting factor in the skin of frogs, it seemed 

 necessary to discover whether the ovisac eggs contained the factor, for 

 it was possible that the egg factor might be merely a contaminant. 

 Dr. J. F. D. Frazer was kind enough to provide a number of frogs that 

 had been used a few hours before for some physiological experiments 



