CHAPTER TWO 



General Introduction to tne Emnryolo^y 

 or tlie Leopard Fro^, Rana pi pi ens 



The embryology of most of the Anura (frogs and toads) is essen- 

 tially the same. However, since the leopard frog, Rana pipiens, is so 

 abundant and is most generally used for embryologicai, physiological, 

 and morphological studies, the following description will be specifically 

 of this form. Where there are differences in closely related forms that 

 may be used in embryology, those differences will be indicated in the 

 text. 



Rana pipiens (Schreiber) has a widespread distribution over entire 

 North America. It hibernates in marshes or pools and seems to prefer 

 the swampy marshlands for breeding in the spring. It may be found in 

 hay fields where there are many insects, but it remains close to a supply 

 of relatively calm water. 



When these frogs are sexually mature they measure from 60 to 

 110 millimeters from snout to anus, the female being about 10 mm. 

 longer than the male of the same stage of maturity. The body is slender 

 and the skin is smooth and slimy, due to a mucous secretion of the 

 integument. The general color is green, except when the animal is 

 freshly captured from hibernation. At this time the chromatophores 

 are contracted by the cold and the frogs have a uniform light brown 

 color. Extending backward from the eyes are a pair of light colored 

 elevations known as the dorsal plicae, between which are two or three 

 rows of irregularly placed dark spots. Each spot has a light (gen- 

 erally yellow) border. On the sides of the body these spots are usually 

 smaller and more numerous, and on the legs they are elongated to ap- 

 pear as bands. Occasionally one may find a dark spot on the tip of 

 each eyelid. Sometimes a light colored line, bordered below by a dark 

 stripe, occurs along the jaw and extends posteriorly below the tympanic 

 membrane and above the forearm. The ventral aspect of the body is 

 always shiny and white. 



The period of germ cell formation occupies much of the long in- 



17 



