238 CLAPP. [Vol. XV. 



He describes a general cutaneous component and a special 

 cutaneous or lateral line component, thd dorsal branches of 

 which innervate the organs of the lateral line. These branches 

 are coarse fibered and therefore distinguishable in sections, 

 while their internal origin or termination is the tuberculum 

 acusticum, a portion of the medulla which is greatly developed 

 in fishes. If the lateral line component has its origin in the 

 tuberculum acusticum, we have good reason to conclude that 

 the localization of function in the medulla is as definite for 

 these widely distributed organs as it is for the more circum- 

 scribed patches of sensory epithelium seen in the case of the 

 ear, eye, or olfactory organ. 



The ear furnishes a fine illustration of this subject, and seems 

 like a connecting link between the system of lateral line 

 organs from which it has probably originated, and the most 

 highly modified sensory structure in Vertebrates — the eye. 

 Ayers (10) has shown that the auditory organ is in reality a 

 series of canal organs innervated by two distinct cranial 

 nerves which he regards as possibly dorsal roots of VII and IX. 



II. Larval Forms. 

 Origin of t lie Lateral Line System. 



I. Lateral line sense organs. — Sections of early stages in 

 the development of Batrachus show thickenings of the ectoderm 

 in the region behind the eyes. In sections of later stages these 

 areas of thickened ectoderm have become invaginated to form 

 the auditory vesicles. Immediately after the closure of the 

 auditory pits, thickenings of the lower layer of the ectoderm 

 are observed on each side of the head in the pre-auditory region. 

 From this thickened area two cords extend, one above and 

 another below the eye. These cords are the rudiments of the 

 supraorbital and infraorbital lines of sense organs. 



Simultaneously in the post-auditory region there appear 

 similar thickenings of the ectoderm which extend rapidly 

 backward on to the trunk. In very young embryos this line 

 advances along the side of the body with an enlargement at the 

 growing end. 



