THE NERVOUS SYSTEM 



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appear, corresponding with fore-brain, mid-brain, and hind-brain, the fore- 

 brain differing from the other two in being anterior to the notochord. 

 From these, by processes of local unequal growth, all the parts of the 

 definitive brain are differentiated. Experiments demonstrate that the 

 position of the three brain divisions is predetermined in the open neural 

 plate. Soon after their closure and expansion as mid-brain and hind-brain 

 vesicles, the lateral walls become divided by a longitudinal sulcus into a 

 ventral basal plate and a dorsal alar plate. Less clearly seen is a narrow 

 floor plate in the mid-ventral line, and a roof plate in the mid-dorsal line. 

 Since this sulcus does not develop in the fore-brain, and ends where the 

 notochord ends, it appears that the fore-brain consists of alar and roof 

 plates only. By the time a human embryo is a month old, the primitive 

 fore-brain vesicle has begun to divide into the anterior telencephalon 

 and posterior diencephalon. In a five-weeks embryo, the hind-brain has 

 begun to divide into the anterior metencephalon and posterior myelen- 

 cephalon. The undivided mid-brain is the mesencephalon. These five 



Brain Vesicles and Their Derivatives 

 (After Keibel and Mall) 



