68 GEORGE L. STEEETER 



It has long been known that in elasmobranchs the endolym- 

 phatic appendage opens directly on the surface of the body and 

 that the surrounding sea-water can thereby pass directly through 

 the endolymphatic duct to the cavities of the labyrinth. The 

 arrangement that we have referred to as existing in the tadpole, 

 suggests that we have there quite a different source of access for 

 the endolymph. At any rate, it is evident that the contact ex- 

 isting between the endolymphatic sac and the membranous roof 

 of the hind-brain affords favorable structural conditions for an 

 interchange of substances between the cerebro-spinal fluid and 

 the endolymph, either by diffusion or by a secretory activity 

 of the separating epithelial membranes. The endolymphatic ap- 

 pendage also in the human embryo serves as an absorption-appa- 

 ratus or one for regulating the endolymph, that is, if we may 

 judge from its structural and topographical characteristics. The 

 condition, however, in human embryos becomes somewhat more 

 complicated than that in the tadpole in that here the sac is sepa- 

 rated very early from the chorioidal membrane by the develop- 

 ment of the dura mater and the intervening arachnoid-pial 

 membrane. Instead of attaching itself to the membranous roof 

 of the hind-brain, the sac projects against one of the large veins 

 of the dura mater. Furthermore, it does not apply itself directly 

 against the vein wall, but is separated from it by an intervening 

 capillary plexus, which in turn drains into the vein. As far as 

 the writer knows the character and connections of this endo- 

 lymphatic capillary plexus is described here for the first time. 

 As to its functional significance we must for the present limit 

 ourselves to the above suggestion and in the following paper 

 attention will be directed only to its morphology as seen in the 

 typical stages of its development. 



MATERIAL AND METHODS 



The specimens which were examined microscopically in con- 

 nection with this study consist of a group of human embryos, 

 measuring from 21 mm. to 240 mm. (crown-rump) long, that is, 

 from about the eighth to the twenty-eighth week of fetal life. 

 They all belong to the Cjollection of the Department of Embry- 



