FROM TWELVE TO THIRTY-SIX SOMITES 147 



III. The Nervous System 



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 (Fig. 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 7ieuropore, 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, i.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 



