CHAPTER XI 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



In discussing the histogenesis of the nervous tissue we have described the 

 early development of the neural tube, as an infolding of the neural plate and a 

 closure of the neural groove (Fig. 297). The groove begins to close along the mid- 

 dorsal line near the middle of the body in embryos of 2 mm. and the closure 



Mesencephalon 



Rhombencephalon 

 Myelencephalon 



Amnion (cut) 



Mesodermal segment 14 



Neural tube (not closed) 



Prosencephalon 



Stomodceum 



Amnion (cut) 



Yolk-sac 



Body-stalk 



Fig. 306.— Human embryo of 2.4 mm. showing neural tube closing and the brain vesicles 



(after Kollmann). 



extends both cranially and caudally (Fig. 306) . Until the end of the third week 

 there still persists an opening at either end of the neural tube, somewhat dorsad. 

 These openings are the neuropores. Before the closure of the neuropores, in 

 embryos of 2 to 2.5 mm. the cranial end of the neural tube has enlarged and is 



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