CHAPTER XII 

 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



In discussing the histogenesis of the nervous tissue the early develop- 

 ment 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 in embryos of 2 mm. along the mid-dorsal line, near the 

 middle of the body, and the closure extends both cranially and caudally 

 (Fig. 324). Until the end of the third week there still persists an opening 



Mesencephalon 



Rhombcntephalon 

 Myelencephalon 



Amnion (cut) 



Mesodermal segment 14 

 Open neural groove 



Prosencephalon 



Stomodaum 



Amnion (cut) 



Yolk sac 



Body stalk 



FIG. 324. Human embryo of 2.4 mm., with a partially closed neural tube (after Kollmann)' 



X 36. 



at either end of the neural tube, somewhat dorsad. These openings are 

 the neurc pores (Fig. 330). Before the closure o the neuropores, in em- 

 bryos of 2 to 2.5 mm., the cranial end of the neural tube has enlarged 

 and is constricted at two points to form the three primary brain vesicles. 

 The caudal two-thirds of the neural tube, which remains smaller in diam- 

 eter, constitutes the anlage of the spinal cord. 

 21 321 



