DEVELOPMENT OF THE CEREBELLUM. 489 
The roof of the fourth ventricle is formed for the most part by the thin epithelial 
layer already described as being formed by the expanded mid-dorsal lamina. This does 
not stretch, however, over its entire extent. As we approach the upper part of the 
ventricle, it is seen to become continuous in the region of the isthmus with a thicker 
lamella. This lamella is bounded above by the intercrossing of the two trochlear 
nerves, which marks on the dorsal aspect of the neural tube the place of junction 
between the rhombencephalon and the mesencephalon ; below, it is limited by a for- 
wardly-directed semilunar fold of the thin epithelial ventricular roof, which takes place 
into the ventricular cavity at the level of the pontine flexure of the brain. The fold 
so constituted is termed the plica choroidea, seeing that mesoderm is introduced be- 
tween its two layers, 
and this ultimately 
gives rise to the 
choroid plexus of 
the fourth ventricle 
(Fig. 362, D). The 
eBe 
otha luy 
MONTICULUS 1 
DECLIVE 
lamella which forms Ty 
PYRAMID 4 
the roof or dorsal 3 
UVULA 
wall of the ven- 
A Cc 
tricle in front of 
the plica choroidea joe PF aa 
is developed into - ] gst 
S coe, - “LOB CENT; : 
the inferior medul wos eon Lay? ' 
lary velum, the cere- ae Rif yreeuve 
sefceess f, 
bellum, and the oe \g BS) 
superior medullary oe \t Ww 
A ralv 4 eee Pe) 3 WN 
V el um (valve of (Exe PYRAMID 
Vieussens). UVULA 
P MEDULLA | NODULE 
The cerebellar oe 
portion of this 
lamella in mesial 
sections through Fic. 362.—From DRrawincs BY Dr. WALTHER KUITHAN, TO ILLUSTRATE THE 
the brain is soon DEVELOPMENT OF THE CEREBELLUM. 
E 
strongly demar- A. Transverse section through the forepart of the cerebellum of a sheep embryo. 
cated from the ~-B Transverse section through the hinder part of the cerebellum of a sheep embryo. 
C. Cerebellum of a human foetus 17 cm. long. 
medullary vela by D. Median section through the cerebellum of a sheep embryo 5 em. long. 
the great thicken-  E., Median section through cerebellum of human foetus 17 em. long, 
ing it undergoes in 1. Suleus primarius. ; 3. Sulcus between pyramid and uvula. 
a dorsal direction. 2. Sulcus above and afterwards behind 4, Great horizontal fissure. 
: the pyramid. T. Transverse groove in roof of ven- 
At first the thick- r.l. Lateral recess ventricle IV. tricle LV. 
ened cerebellar 
lamella forms a simple uniform arch around the dorsal aspect of the upper part of the 
fourth ventricle. In a short time, however, it becomes thinner along the median plane, 
whilst it increases in bulk on either side (Fig. 362, A). Asa result of this, the cerebellum 
is now seen to consist of two thick plates joined by a narrow and thin median portion. 
This bilateral condition of the organ is transitory. In the further course of development 
the two plates fuse completely in the middle line (Fig. 362, B), and sulci then begin to 
make their appearance on the dorsal aspect of the organ. 
The sulci which come earliest into view are limited to the region of the worm. The 
first sulcus appears between the regions corresponding to the monticulus cerebelli and 
the clivus cerebelli, and in the adult cerebellum this may be seen to be the deepest of all 
the fissures of the vermiform process. It may be termed the suleus primarius (Hig. 
362, C,D, EK). The portion of the vermis which lies below this fissure soon shows other 
sulci. One appears above the region of the pyramid, another between the pyramid and 
uvula, whilst a third marks off the nodule. All this time the portion of the vermis above 
the sulcus primarius remains more or less smooth, but very soon it becomes divided up into 
its constituent parts. The earliest sulcus to appear in the lateral hemisphere is the great 
horizontal sulcus (Fig. 362, C). This at first consists of separate portions in the two 
hemispheres, and it is only in the later months of development that these run together over 
the vermis, so as to separate the tuber valvule from the declive monticuli. Kven at the 
time of birth the folium cacuminis is not seen on the surface. It rises up from the bottom 
of the horizontal sulcus in the form of a secondary folium of the declive monticuli. 
Very early a transverse groove appears on the smooth anterior or ventricular surface 
