THE FROG — A REPRESENTATIVE VERTEBRATE 395 



these only the head and trunk are present in the frog. A neck region is 

 not evident and the tail is lost during embryonic development. 



A large mouth is located at the anterior end of the head and a pair 

 of external nostrils, or external nares, is dorsal to the front of the 

 mouth. The large and protruding eyes are protected by eyelids. The 

 upper one is a simple skin fold; the lower one is a translucent mem- 

 brane. When the eyeball is retracted into the eye socket, the lower lid 

 spreads over its surface. Between and in front of the eyes on the top of 

 the head is a light-colored spot about the size of a small pinhead. It is 

 known as the brow spot, and is a vestige of the median eye of very 

 primitive vertebrates. A round eardrum, or tympanic membrane, lies 

 posterior to each eye. It is noticeably larger in the males than in the 

 females of some common frogs such as the green frog {R. clamitans) and 

 bullfrog {R. catesbiana), but not in R. pipiens. 



The forelegs {pectoral appendages) are much shorter than the hind 

 legs (pelvic appendages) and do little more than hold up the front of 

 the body; the powerful hind legs are the main organs of locomotion. 

 Comparisons can easily be made between the frog's appendages and our 

 own, for they consist of the same parts, but several differences in details 

 will be observed. Only four fingers (digits) are present on the hand of 

 the frog, for the first digit, i.e., the thumb, is missing. The most medial 

 digit (which phylogenetically is the second) is stouter in the males than 

 in the females of many species of frogs, especially during the breeding 

 season, and helps the male to grasp the female. Five digits are present 

 in the foot, the most medial being the first, the equivalent of our own 

 big toe. A membranous web extends between the toes. A small spurlike 

 digit known as the prehallux is located medial to the base of the first 

 typical toe. Two of the ankle bones are elongated, so the foot is very 

 long. This increases the le\erage for jumping and swimming. 



An anus, or cloacal aperture, is located at the posterior end of 

 the trunk. This opening is best called a cloacal aperture in the frog, for 

 a cloaca (a chamber receiving the products of the digestive, excretory 

 and genital tracts) is present. Strictly speaking, the anus is the posterior 

 opening of the digestive tract only. 



181. Skin and Coloration 



The soft, smooth, moist skin, or integument, is more complex than 

 one might suspect. It serves for protection, sensory reception and for gas 

 exchange between the organism and its environment. The integument 

 consists of two layers of tissue— a superficial epidermis and a deeper and 

 much thicker dermis (Fig. 21.2). The epidermis is composed of stratified 

 squamous epithelium, whose basal cells are columnar in shape. These 

 cells proliferate actively by mitosis and this portion of the epidermis is 

 known as the stratum germinativum. Newly formed cells move outward, 

 are flattened through various pressures, accumulate some horny material 

 (only a small amount in frogs), eventually die, and are finally sloughed 

 off in large sheets. The outer, somewhat horny layer of the epidermis is 

 known as the stratum corneum. 



