CHAPTER XII 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



In discussing the histogenesis of the nervous tissue the early development of 

 the neural tube has been described as an infolding of the neural plate (Fig. -78) 

 and a closure of the neural groove (Fig. 304). The groove begins to close along 

 the niid-dorsal Kne, near the middle of the body, in embryos of 2 mm. and the 



Mesencephalon, 



Rhomhencephalon. 

 Myelencephalon 



Amnion {cut) 



Mesodermal segment 14 



Neural tube {not closed) 



Prosencephalon 



Stomodaum 



Amnion {cut) 



Body stalk 



Fig. 324. — Human embryo of 2.4 mm. showing a partially closed neural tube and the brain vesicles (after 



KoUmann). X 36. 



closure extends both cranially and caudally (Fig. 324) . Until the end of the third 

 week there still persists an opening at either end of the neural tube, somewhat dor- 

 sad. These openings are the neuropores (Fig. 330). Before the closure of the 

 neuropores, in embryos of 2 to 2.5 mm. the cranial end of the neural tube has en- 



321 



