12 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



not involving the ovisacs. In the first year, most of these frogs had 

 ovulated, but had not laid their eggs. In the second, none had 

 ovulated. 



The eggs found in the ponds have two envelopes. The ovisac eggs 

 have only one, a firm layer of jelly about 0-3 mm thick. These eggs 

 are, however, embedded in a viscous protein solution, and can move 

 freely and separately in this hquid. If these eggs, with the adhering 

 hquid, are put into water, they immediately stick together and also to 

 any surface that they touch, just as they do when they are laid naturally. 

 Swammerdam, writing in the seventeenth century was probably the 

 first to note this sequence of events, when he wrote, "But as the eggs 

 rendered very clammy and glutinous by the white that invested them 

 have grown together, had been compressed in the uterus, they im- 

 mediately on being cast into the water, expand themselves into their 

 former round form!" (Eighteenth century translation.) 



The ovisac eggs begin to swell at once, and in a few hours resemble 

 in every way the naturally laid eggs, including the two layers of jelly. 

 If, however, the eggs are taken from the ovisacs and put into normal 

 saline, they do not adhere to each other, nor to other surfaces. In fact, 

 an egg can then be rotated in contact with another with unusually httle 

 friction. They remain separate indefinitely, and thus resemble those 

 o£ Discoolossus pictus. The eggs put into saline swell just as they would 

 have done in water (but more slowly), and have only one layer of jelly. 

 If they are thoroughly washed in saline, they do not adhere at once 

 when put into water, but in time they may do so imperfectly. 



It is now obvious that the two layers of jelly are composed of two 

 quite different proteins. The inner is a gel, even within the frog. The 

 outer is a sol that becomes a gel instantly when it comes into contact 

 with water, but not when the water is replaced by saline. These are the 

 properties of the class of proteins known as globulins, and the formation 

 of the outer envelope seems to depend on nothing more than the 

 precipitation of the globulin when the salt that kept it in solution has 

 been removed by the water of the pond. It is an ordinary chemical 

 reaction, and is practically instantaneous. The saline solution of this 

 protein contains no clotting factor, but a saline extract of the inner 

 envelope contains a factor that behaves just as that reported by Dr. 

 Barnett. The activity of these extracts is quite small, but that is not 

 surprising, when it is considered that most clotting factors are unstable 

 and disappear soon after they have done their work, and that the 



