118 CARNEGIE INSTITUTION OF WASHINGTON. 



appendage, and one is on the development of the tissue-spaces sur- 

 rounding the membranous labyrinth. 



The development of the endolymphatic appendage in the turtle 

 {Chrysemys marginata) has been described by Professor Keibel. His 

 observations refer particularly to the relation existing between the 

 appendage and the area at which the ear-vesicle finally becomes 

 detached from the skin. This problem involves the homology of this 

 appendage as it is found in the different vertebrates, and concerning 

 which there has been active discussion during the past few years. 

 Professor Keibel maintains that the place at which the ear-vesicle 

 detaches itself from the skin has a direct bearing on the question of the 

 homology of the ductus endolymphaticus — a view contrary to that 

 generally expressed by other recent writers. He finds the ductus 

 endolymphaticus to be an homologous structure in amphibians, 

 reptiles, birds, and mammals, although it is not always a morphological 

 equivalent. In these fonns (such as Anura and Ascalahota in which 

 certain portions of the endolymphatic appendage undergo special devel- 

 opment) such parts early make their appearance as projecting pouches 

 peripheral to the point of separation from the skin. These parts have 

 no homologues in selachians, but already exist in them in an indifferent 

 condition, as a portion of the ductus endoljTiiphaticus. 



Some interesting features in the development and topography of 

 the endolymphatic appendage in the human embryo have been pub- 

 lished by Professor George L. Streeter. An important capillary plexus 

 is associated with the appendage, the character and communications 

 of which are described by Dr. Streeter for the first time. He finds 

 that throughout the greater part of fetal life the endolymphatic 

 appendage is ensheathed in a vascular plexus, the plexus endolymphat- 

 icus, which anastomoses on the one hand with the vessels of the rest 

 of the labyrinth, and on the other with the transverse sinus, into which 

 it drains through several openings. This plexus makes its appearance 

 at about the time of the differentiation of the appendage into its adult 

 subdivisions of duct and sac. It can be plainly recognized in embiyos 

 30 mm. long. In embryos 50 mm. long it is well-developed and at that 

 time forms a closely-meshed web completely investing the appendage, 

 whereby the latter is virtually inclosed in a sheet of blood, from which 

 it is separated only by the endothelium of the blood-spaces. In the 

 course of its further enlargement and development in embryos 100 mm. 

 long and over, the endolymphatic plexus becomes resolved into a few 

 principal channels, connected with which there remain parts of the 

 original plexus. The plexus persists notably in the neighborhood of 

 the endolymphatic sac. One of the most constant channels developed 

 through the endolymphatic plexus is the one forming the so-called 

 vena aquajductus vestibuli. This fonns along the side of the endo- 

 lymphatic duct and the posterior margin of the endolymphatic sac and 

 constitutes a direct communication between the vascular plexus sur- 



