584 THE EMBRYOLOGY OF THE FROG 



There may be also a supratemporal line, connecting the systems of 

 both sides and extending across the posterior portion of the skull from 

 one side to the other. 



These lateral line organs sink beneath the skin, and usually degen- 

 erate in water-forms of animals as soon as they are ready to live on 

 land. In a few cases such as Tritons, Amblystoma, etc., they are said to 

 reappear when the animals return to water to deposit their eggs. 



Various functions are assigned to these sensory line organs, but 

 none has been clearly demonstrated. Probably they assist in recognizing 

 differences in the vibrations of the 'water and may permit the animal to 

 determine currents. They have been called a "sixth sense." 



In the frog tadpole the sense organs of the lateral line are derived 

 from the placode of the X cranial nerve. The ramus lateralis of the X 

 nerve innervates the organ. 



When the embryo is about four millimeters in length, there is a 

 small dorso-lateral section of the vagus ganglion separated from (though 

 lying close to) the ectodermal placode. The placode then begins to 

 elongate posteriorly, while the deep cells proliferate rapidly to form a 

 long, narrow tongue, which then pushes through the epidermis just out- 

 side the basement membrane. This tongue reaches as far back as the 

 tip of the tail by the time of hatching. 



Along this line, groups of cells form at intervals, each group rep- 

 resenting the beginnings of a definite sense organ of the lateral line. In 

 each group there are a few central sensory cells surrounded by a layer 

 of enveloping cells. These groups then push up through the epidermis 

 to the surface of the body, and the sensory cells develop hair-processes. 



There are other tegumentary sense organs developing in definite 

 rows on the head as well as dorsally from the mid-line to form those men- 

 tioned above, which are innervated by twigs from the VII, IX, and X 

 cranial nerves. 



All of the lateral line organs disappear, however, when the tadpole 

 becomes an adult frog. 



