358 
Cephalad to the depression (f) the neural axis is divided into 6 
segments by the presence of transverse grooves. These are fainter 
but otherwise resemble the grooves that divide the medulla into joints. 
There are thus in all 11 encephalic segments present that differ from 
one another in no essential feature. Their number is constant in all 
the embryos examined. If a metameric value is assigned to those of 
the medulla, the same interpretation should be placed upon the joints 
that lie cephalad to this region. 
In later stages the posterior commissure develops at the joint (c) 
and as segment 6 forms the cerebellum, we are able to say that seg- 
ments 1, 2 and 3, represent the fore-brain, and that segments 4 and 
5, will produce the mid-brain. Fore-brain and mid-brain are thus 
compound structures formed from an already segmented axis. They 
have frequently been erroneously considered as homologous divisions 
with the metameric joints of the medulla. (Miss PLATT, NEAL and 
others.) 
In a living embryo two days older (Fig. 2) the 11 encephalic 
segments are present as described in Fig. 1, but the dividing grooves 
are deeper and can be traced down the sides of the neural axis. The 
first indication of a neural canal has appeared as a fine median cleft 
dividing the neural axis into lateral halves. The dorsal groove (f) 
marks the posterior border of the cerebellum. A lateral neural ex- 
pansion has taken place in its vicinity, which is soon covered by a 
thin transparent roof. 
The segments of the medulla in this embryo could be traced com- 
pletely around the neural tube. In like manner segments 5 and 6 
could be traced into the ventral zone, the latter the cerebellum and 
the former the posterior segment of the mid-brain. The anterior 
limits of segments 2, 3 and 4, were marked only by dorsal constric- 
tions but in dissected embryos 19 days and older, the anterior limits 
of these segments can be clearly traced into the ventral and lateral 
zones, and my failure to trace them there in living embryos is prob- 
ably due to the presence of a dense layer of mesoderm in this region 
and to the position of the lateral eyes. 
Fig. 3 represents the dorsal view of the cephalic portion of a 
living embryo with 35 somites, 23 days old. Five segments are clearly 
present in front of the cerebellum. Their limits are marked by in- 
ternal and external transverse constrictions that appear to lie in the 
same transverse plane. The internal constrictions are deeper, or in 
other words, the segmentation is more pronounced on the internal 
aspect of the brain-wall than on the external surface. 
