FROM TWELVE TO THIRTY-SIX SOMITES 147 
Ill. THE Nervous SystEmM 
The Brain. The description of the nervous system in the pre- 
ceding chapter forms our starting-point. During the period now 
under consideration the foundation of the main parts of the adult 
brain are laid down, and its five chief divisions become sharply 
characterized. It is important to correlate these with the earliest 
morphological characters (original anterior end of medullary 
plate, neuromeres, etc.) in order to trace these fundamental 
landmarks through to definitive structures. 
As we have already seen, the primary fore-brain includes the 
first three neuromeres, the mid-brain the fourth and fifth, and 
the hind-brain the sixth to the eleventh, as well as the region 
opposite to the first four mesoblastic somites. It is clear that a 
second point of fundamental morphological significance is the 
original anterior end of the medullary plate which would naturally 
form the center for a description of the anterior part of the neural 
axis, if recognizable throughout the development. This point 
may be recognized for a considerable period after the closure of 
the anterior part of the neural tube, as the ventral end of the 
anterior cerebral fissure (ig. 62), opposite the center of the 
primary optic vesicles, thus in the region of the recessus opticus 
(Figs. 87 and 88), which is to be regarded as marking the original 
anterior end of the neural axis. Even after closure of the anterior 
cerebral fissure a connection remains at its dorsal end between 
the ectoderm and the neural tube. To this we may apply the 
name neuropore, though no actual opening is found here at this 
time. The median stretch of tissue between the recessus opticus 
and the neuropore constitutes the lamina terminalis which remains 
as the permanent anterior wall of the neural tube. It must not 
be forgotten that the original anterior end of the medullary plate 
lies at the ventral end of the lamina terminalis, 7.e., in the re- 
cessus opticus. <A third landmark of fundamental morphogenic 
significance is the infundibulum, which coincides in position, as 
we have seen, with the anterior end of the notochord. Thus we 
may distinguish prechordal and suprachordal portions of the neu- 
ral axis (cf. Fig. 67). 
Dorsal and Ventral Zones in the Wall of the Brain. The con- 
ception of His, that the walls of the neural tube may be consid- 
ered as formed of four longitudinal strips, viz., floor, roof, and 
