1^4 AVERS. [Vol. VI. 



but the majority of the facts of the development of this com- 

 plexus have been so often studied and the results of the earlier 

 observers so often confirmed that as regards the mode of devel- 

 opment there can be little question. It will be noticed, I think, 

 in the accounts which I shall give of the ontogeny of the ear 

 in the various types, and which I shall endeavor to make 

 as complete as possible, that there is a remarkable absence of 

 any attempt to account for the phylogeny of the vertebrate ear 

 with the facts derived either from ontogeny alone or from com- 

 parative anatomy and development combined. 



The ontological history of the lateral line organs of Sharks 

 and Skates is up to the present but partially recorded, and the 

 subject is one demanding thorough investigation, both from its 

 intrinsic interest and the very great importance of its relation 

 to the morphology and physiology of sense organs in general. 

 I have not the necessary material for a complete history, and 

 shall attempt here only a brief sketch of some of the important 

 stages of the development of the lateral line organs of the head, 

 in so far as they have a direct bearing upon the question of ear 

 production and an anatomical connection with the specialized 

 canal complex known as the internal ear. The early stages of 

 these organs have been studied by many investigators, notably 

 Balfour, Beard, Mitrophanow, and Semper. The later stages 

 have been almost completely neglected. As is well known, the 

 auditory thickening, soon converted into a cup or pit (PI. I, Fig. 

 6, Cuts 17, 18, 19), appears long before the other parts of the 

 lateral line system are established, but as it sinks below the 

 surface it retains its connection with the surface of the body by 

 means of a canal, the endolymphatic duct of later stages. This 

 canal opens out into a groove on the surface of the head which 

 is lined by a peculiar and characteristic epithelium. Owing to 

 the concomitant growth of the head and the closure of the 

 auditory groove, the first of which is the main cause, while the 

 latter is mainly a result of the transposition of the ear complex 

 already well below the surface of the head and surrounded by 

 a semi-fluid mesodermic tissue, the ear hole, or orificium ductus 

 endolymphatici, travels forward, the epithelium of the groove 

 increasing to an extent sufficient to maintain the anatomical 

 connection of the sunken ear with the sensory epithelium on 

 the surface of the body, which later events prove to have only 



