CLASS AMPHIBIA 371 



rally frogs avoid a degree of heat which would cause their skin to become 

 dry. 



Frogs can form simple habits, although they do so very slowly. 

 Yerkes found that after about a hundred trials a frog was able to traverse 

 the proper path in a simple labyrinth of passages. Some intelligence 

 may have been involved in this behavior, but it was, apparently, largely 

 the formation of a habit. 



400. Reproduction and Development. — All Amphibia are diecious. 

 The eggs are set free in the body cavity of the female and are collected 

 by the open ends of coiled oviducts. They are accumulated in thin- 

 walled distensible portions of the oviducts known as uteri. The glands 

 of the oviducts secrete the gelatinous coating of the eggs. Certain 

 facts have been stated in regard to the reproduction of particular forms, 

 but the development of the frog will be given in detail as typical of the 

 class. 



Frogs deposit their eggs in water in the spring. AVhile in the body 

 of the female the eggs are surrounded with a layer of transparent jelly 

 which is thin, but as soon as the eggs are brought in contact with the 

 water this jelly absorbs water and swells, becoming thick and serving 

 as a protective covering. During egg laying the male clasps the body of 

 the female by his forelegs and fertilizes the eggs by depositing sperm 

 cells upon them as they are passed out of the cloaca. The upper pole 

 of the egg, called the animal pole, is dark in color, and the lower, or 

 vegetal pole, is light because of the massing of the yolk in that portion. 

 The eggs are holoblastic but, owing to the amount of yolk, undergo 

 unequal cleavage (Fig. 253). The upper and smaller cleavage cells 

 are known as micromeres, the lower and larger ones are the macromeres. 

 A blastula cavity, or blastocoel, is formed, and gastrulation takes place 

 by epibole, a fold of micromeres growing around and inclosing the 

 macromeres, leaving the yolk visible only through the blastopore. This 

 visible portion of the yolk is termed the yolk plug. Soon after gastrula- 

 tion there is developed a groove, called the medullary groove, running 

 dorsally from the blastopore forward toward what will become the 

 anterior end of the larva. The blastopore gradually becomes obliterated 

 by the contraction of its margins. The embryo now becomes elongated 

 and the head and tail become free. 



Later, and after the embryo has become better developed, a swelling 

 appears on each side near the anterior end of the body. Below each 

 swelling is developing a gill arch, and in front of it a depression which 

 moves toward the ventral side of the body and unites with that of the 

 other side to form a ventral sucker. Above the ventral sucker an invagina- 

 tion called the stomodeum marks the beginning of the mouth, while 

 toward the posterior end of the body, below the tail, which is developing 

 backward, is formed another invagination, the proctodeum, which will 



