70 GEORGE L. STREETER 



rinth; In this instance the ear vesicle remains attached to the 

 skin throughout the whole period of its development. In 

 other vertebrates it persists only as an embryological remnant of 

 varying size that terminates as a blind sac under the dura mater 

 and is apparently of no further use (Balfour '81, Hoffman '90, 

 Hertwig '98). 



This was the prevailing view regarding the endolymphatic 

 appendage until results that conflicted with it were reported by 

 Poll '97 and Netto '98. These investigators found that in rep- 

 tiles and amphibians it is the lateral surface of the ear vesicle that 

 is last to be detached from the skin, at a place clearly remote 

 from the dorsal tip that gives origin to the endolymphatic duct. 

 It was also found that in some cases the endolymphatic append- 

 age does not make its appearance until after the detachment 

 from the ectoderm is completed. Keibel '99 was strongly influ- 

 enced by the condition existing in the embryo of the chick, 

 where the separation of the otic vesicle from the ectoderm occurs 

 relatively late and in fact the last point of attachment does occur 

 at the dorsal tip of the endolymphatic appendage, and he there- 

 fore supported the original view of Balfour '81. He quite cor- 

 rectly defends the opinion that the tube in Selachians connect- 

 ing the inner ear with the ectoderm is the same as the endo- 

 lymphatic duct of the vertebrates. The conditions found in 

 amphibians by Netto '98, where the endolymphatic duct does 

 not develop until a considerable time after the complete detach- 

 ment of the ear vesicle, he explains as a shifting in the time of 

 occurrence of the ontogenetic as compared with the phylogenetic 

 processes. 



Subsequently the origin of the endolymphatic sac was care- 

 fully reviewed by Krause '01 who had an abundance of material 

 for a comparative anatomical study. He showed that in reptiles 

 the point of separation of the ear vesicle from the ectoderm has 

 nothing to do with the dorsal pointed end of the vesicle from 

 which the endolymphatic duct arises. While in birds, as de- 

 scribed by Keibel '99 and others, it corresponds exactly to the 

 tip of the endolymphatic duct. In mammals it also corresponds 

 approximately to the tip of the endolymphatic duct, but here the 



