THE EARLY DEVELOPMENT OF THE FROG 



81 



to rotate within its membranes, and in a few moments after 

 fertilization all the eggs are found with the pigmented pole 

 uppermost. In unfertilized eggs these membranes are more 

 adherent and while rotation occurs, it is very slow. 



Probably some of the fluid in the perivitelline space is taken 

 in from the outside, for the egg membranes, particularly the 

 jelly, are extremely hydroscopic. The egg has been in the 

 water only one minute when the thin jelly has visibly com- 

 menced its absorption of water. When the eggs are extruded 



FIG. 28. Egg of frog a short time after laying and fertilization, showing the 

 swollen egg membranes. From Ziegler (Lehrbuch, etc.), after O. Schultze. 

 mb, The so-called vitelline membrane; p, pigmented penetration path of the 

 spermatozoon; r, polar bodies; 1, 2, 3, inner, middle and outer albumenous 

 membranes or layers of "jelly." 



the thickness of the jelly is only about one-sixth the diameter 

 of the egg; after three minutes contact with the water this is 

 increased to one-half the diameter; and after ten to fifteen 

 minutes its thickness equals the diameter of the egg. The 

 swelling then becomes slower and unless fertilization has 

 occurred it may almost cease. Usually however the absorp- 

 tion of water continues for several hours and the thickness of 

 the jelly may equal twice the diameter of the egg proper. 



As the jelly thickens it is seen to be arranged in definite 

 strata a thin denser layer closely applied to the chorion, out- 

 side this a thick layer somewhat more fluid, and on the sur- 



