DEVELOPMENT OF THE SACCUS ENDOLYMPHATICUS 245 
The dorsal extension of the saccus endolymphaticus is illus- 
trated in figure 14. In its anterior part only an increase in size 
can be observed. In the posterior part, however, a great change 
has taken place. At the end of the fourth ventricle the sacci 
endolymphatici of the two sides grow towards each other and 
meet in the median line (fig. 7). From the point of juncture the 
united sacci run caudally as far as the seventh vertebra. 
Figure 18 shows the lateral expansion of the saccus endolym- 
phaticus. In this stage the processus ventralis can be seen 
(fig. 18, pr.vent.). It starts from that part of the saccus stem 
which hes lateral to the lobi optici and dorsal to the ganglion 
prooticum commune. The processus extends ventrally as far 
as the lateral surface of this ganglion. 
In the region of the foramen endolymphaticum the saccus 
endolymphaticus is an undivided sac. Directly in front of and 
behind this aperture, however, it divides into several smaller 
tubul. In the cranial part four tubuli are formed, which run 
independently of one another into the region of the hemispheres, 
where they coalesce. The caudal part of the saccus remains 
undivided to the end of the fourth ventricle. At the place where 
the sacci endolymphatici of the two sides join, each saccus divides 
into two tubuli (fig. 7, t.s.e.) which run parallel to each other, 
and, towards their ends, split up into many small tubuli. The 
division sometimes takes place in a horizontal direction, some- 
times in a vertical one. 
Each of the tubuli is lined with the characteristic epithelium 
of the saccus endolymphaticus and is embedded in a delicate 
structure of connective tissue. The tubuli are surrounded by 
many blood vessels. Between the two partes spinales runs the 
vena spinalis dorsalis duralis (fig. 7, v.sp.d.). At the place 
where the two partes spinales separate, this vena divides into 
two branches, which run toward the front in the outer walls of 
the sacci and finally lead into the foramen trigemini. Both the 
stem and the two branches collect all the veins of the saccus. 
The lumen of the saccus endolymphaticus is filled with cal- 
careous matter. 
