DEVELOPMENT OF THE SACCUS ENDOLYMPHATICUS 239 
The saccus endolymphaticus, though larger than in stage I, 
still remains, in contrast to the labyrinth and ductus, in a 
rudimentary stage, and does not arrive at the complicated form 
which it has in the adult animal until much later. It lies on the 
roof of the fourth ventricle (fig. 3, vent. 4) extending a little 
eranially and caudally beyond the foramen endolymphaticum. 
Its lumen is largest in the region of this opening and tapers 
slightly cranially and caudally. 
The position of the saccus endolymphaticus in relation to the 
spinal meninges has been much discussed. In the first place, 
the descriptions of these membranes have not been uniform. 
Hasse (’73) and Rex (’93) mention three meninges: 1) the dura 
mater, which closely invests the bones; 2) the arachnoidea, 
separated from the dura by the subdural space, and, 3) the pia, 
closely enveloping the brain and the spinal cord. According 
to the opinion of the above-named investigators, the saccus 
endolymphaticus is situated in the subdural space between the 
dura and the arachnoidea. Sterzi (’99) has a different view. 
He terms the membrane immediately investing the bone, the 
endorhachis, and says that ventral to this les the dura mater, 
whereas the innermost membrane is the arachnoidea. Accord- 
ing to Sterzi, the saccus endolymphaticus would lie between the 
endorhachis and the dura mater, separated from the endorhachis 
by the epicaleary space, and from the dura by the epidural 
space. Gaupp (04), Coggi (90), and O’Neil (98) describe the 
membranes as follows: 1) the dura mater, which is divided dor- 
sal and lateral to the brain and the spinal cord into two lamellae, 
one of which closely invests the bone, while the other lies more 
ventrally, and, 2) a primary vascular membrane, in which a pia 
and an arachnoidea are as yet not distinctly differentiated. The 
lymphatic space between the two lamellae of the dura is called 
the interdural space, and that between the inner lamella of the 
dura and the vascular membrane the subdural space. Accord- 
ing to these observers, the saccus endolymphaticus lies in the 
interdural space. My investigations confirm this view in every 
respect (figs. 3, 4, ect.p.d.m. and n.d.m.). As O’Neil describes 
in detail and I was able to verify, the dura mater is divided in 
