THE PHYSIOLOGY OF AN ANIMAL 215 



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their action, the eggs are swept into the opening, and then 

 slowly pass down through the coils of the oviduct toward the 

 uterus. As they pass along they are covered with a gelati- 

 nous substance, which is secreted from the glands in the 

 walls of the ducts and forms a layer around the eggs. When 

 the eggs reach the uterus they are stored there for a time until 

 the animal is ready to lay her eggs. 



With the approach of the breeding season the spermaries of 

 the male also become very active and secrete sperm fluid. This 

 passes down the ducts to be stored in the seminal vesicles, where 

 it remains until the period of copulation. 



At the breeding season the male frog fastens himself to the 

 female, who is about to lay her eggs, and remains firmly attached 

 to her until she lays them, remaining thus attached for days or 

 even for weeks in some cases (copulation). After the eggs are 

 laid the male leaves the female and pays no further attention 

 to her. When the eggs are laid they are rather slowly passed 

 from the body by the cloacal opening, and at the same time 

 the male ejects the sperm fluid from his body over them. The 

 sperms themselves penetrate the jelly and eventually enter the 

 eggs, producing fertilization. After the eggs are thus laid the 

 ovaries and the oviducts contract and in a short time shrivel to 

 a size much smaller than that which they had at the "breeding 

 season. This diminished size continues until late in the summer, 

 when the ovaries begin to increase in size again with the growth 

 of the ova, in preparation for the next breeding season. 



The eggs of the common frog are always laid in water and 

 at first form a rather small mass of eggs with their surrounding 

 j elly . But the j elly quickly absorbs the water and swells to many 

 times its original size, inclosing each egg in a thick layer. This 

 jelly appears to have two purposes. It is a protection to the 

 eggs from the attack of birds and perhaps other enemies. It 

 seems also to have the power of absorbing the sun's rays and 

 holding them back from too great radiation, the result being that 

 the egg is kept warmer than it would be without the jelly. This 



