The Development of the Frog 7 



chiefly due to the large amount of food-yolk 

 contained in the frog's egg (Fig. 3.) 



As the eggs ripen and are set free from the 

 ovary, they fall into the body cavity and pass 

 forward into the abdominal openings of the 

 oviducts. Passing slowly along these ducts, 

 the eggs at last collect in large numbers in the 



Fig. 3. — Ovarian egg of frog. 



thin-walled, dilated posterior parts of the ovi- 

 ducts, where they remain until they are forced 

 out into the water at the time of spawning. 



Owing to the opacity of the frog's egg, the 

 nucleus is only visible in sections, although it 

 is very large, sometimes as much as one-third 

 to one-half the diameter of the egg (Fig. 3). 



Before it is set free from the ovary, the egg 

 has secreted around it, in some way that is not 

 well understood, a thin vitelline membrane ; 



