CLASS AMPHIBIA 377 



very small size of the cerebellum seems to show that it has no important 

 function. After all of the brain is removed except the medulla the 

 animal still continues to breathe, will snap at food brought in contact 

 with its jaws, is able to leap, swim, and right itself when placed upon 

 its back. Destruction of the medulla, however, results in death. This 

 clearly shows that the vital centers are lodged in the medulla and that 

 the actions of the frog are very largely reflex in character. 



Since it is true that the activities of the frog are mainly reflex it is 

 also clear that they are governed largely by instinct. The fact that 

 the removal of the anterior part of the brain in front of the medulla has 

 so little effect upon its activities indicates plainly the low grade of 

 intelHgence possessed by the animal. The roof of the cerebrum in all 

 forms up to Amphibia has been epithelial and without nerve cells. In 

 Amphibia it contains nerve cells, but these are inside and are covered by 

 fibers and are not organized into a cortex. 



The frog responds directly to many external stimuli. It is sensitive 

 to light, the whole skin being affected. The animal is said to exhibit a 

 negative phototropism since it avoids bright light and, when exposed 

 to it, faces it, that being the position in which the smallest amount of 

 light will be received by the skin of the body as a whole. Frogs are 

 also stimulated by contact and tend to crawl under objects and into 

 crevices. Both of these responses are modified by temperature. Natu- 

 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 intelhgence 

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

 the formation of a habit. 



402. 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. While 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 



