THE EGGS AND YOUNG TAUl>OLF.S I3 



substrate used for testing was not that natural to the system. It seems 

 almost certain that all that was found was the remains of a powerful 

 clotting system, functioning within the oviduct. The probable course 

 of formation of the inner envelope is this: the secretion of the oviduct 

 is clotted by the action of tliis factor, and the vitelli, as they pass down 

 the duct, act as nuclei for the deposition of the clot, and thus become 

 invested by a spherical shell of jelly. Rugh (1935) lias found that lead 

 shot will become coated with jelly if it is introduced into the oviduct. 

 Tliis view is also supported by the observation that, in all the frogs 

 examined, the ovisac eggs included some spheres with no vitelli inside 

 them. These spheres were in other respects just the same, and swelled 

 in the same way. The clotting factor docs not come from the vitelh, 

 and this conclusion was made certain by the examination o(^ the clump 

 of jelly mentioned above as having no vitelli m it. It contained the 

 clotting factor. 



In the second season, when none of the frogs had ovulated, an 

 attempt was made to fmd the source of the factor. The coelomic fluid 

 was inactive, and so was a saline extract of fmcly divided oviduct and 

 ovary. The resemblance to blood-clotting became even closer, for 

 just as blood thromboplastin appears as if by magic just when it is 

 wanted, so does the clotting factor for the egg-jelly. The analogy 

 must, however, not be pushed too far, for the biochemistry is probably 

 quite different in many ways. This is a biochemical problem that may 

 remain obscure for a very long time. It may be as comphcatcd as 

 blood-clotting, and the scientific effort required for its solution is never 

 likely, in the foreseeable future, to approach that devoted to 

 haematology. All that I have done is to hft a htde higher a corner 

 of the curtain against which Dr. Barnett "stumbled." 



Ovisac eggs swell even in 10 per cent saline and 30 per cent urea, 

 solutions with osmotic pressures far above that of frog plasma. The 

 swelling is undoubtedly due to the imbibition of water by a fairly 

 concentrated protein gel, a process that has nothing to do with osmotic 

 pressures. A puzzle that still remains is this: why do the eggs not 

 swell while they are inside the frog? Actually, they do swell sometimes, 

 and Nussbaum (1897) says that if oviposition is delayed the frogs may 

 die. It is possible that the reason this does not usually happen is simply 

 that there is not often time for the absorption of enough water to be 

 dangerous. Oviposition usually takes place at most a few days after 

 ovulation, and the eggs form such a large proportion of the weight 



