78 OUTLINES OF CHORDATE DEVELOPMENT 



later in the spring, and in a few (e.g., R. catesbiana) , breeding 

 occurs during early summer. In the first mentioned, spawning 

 follows immediately upon emergence from the period of hiber- 

 nation, when the frogs collect in small ponds or streams, or 

 about the margins of larger bodies of water. There is no true 

 copulation, the male merely seizing the female firmly around 

 the body dorsally, with the forelegs just behind those of the 

 female. This embrace or amplexus usually begins some hours, 

 even days, before the actual extrusion of the reproductive 

 products begins, and quite likely this affords the normal, 

 though not essential, stimulus to their discharge from the 

 ovaries and testes respectively. This amplexus continues 

 throughout the entire period of spawning of a single pair, 

 which may occupy several days or even weeks; the duration 

 depends upon the species and upon the temperature colder 

 weather prolonging the period greatly. 



Expulsion of the eggs usually occurs during the early morn- 

 ing hours and is an intermittent process. Apparently all the 

 eggs contained in the uteri are spawned at one time, and then 

 an interval of rest follows during which the uteri are again 

 slowly filled. As each mass of eggs is forced out of the cloaca 

 the male, at the same instant, expels quantities of seminal 

 fluid containing enormous numbers of spermatozoa which 

 mingle with the egg masses, insuring the fertilization of practi- 

 cally every egg. Fertilization is therefore strictly external. 



In the common frogs there are no nursing habits so frequent 

 among other Anura (e.g., Alytes, Nototrema, Rhinoderma, etc.) 

 and the eggs are left to develop without further relation to the 

 parent organisms, which, upon the conclusion of spawning, 

 immediately leave the pools and scatter widely. The eggs 

 surrounded by the jellies remain in large masses which sink 

 to the bottow of the shallow water and there become loosely 

 attached to sticks or debris. 



The total number of eggs laid by a single individual during 

 one season varies widely in different species, and seems to vary 

 conversely with the size of the eggs. The European grass- 

 frog (Rana temporaria) lays from one to two thousand large 



