402 EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



and another in the posterior primary vesicle, dividing each into 

 two and making in all five secondary brain vesicles, which freely 

 communicate with one another and are numbered from before 

 backward. They are: 



1. Telencephalon. 



2. Diencephalon. 



3. Mesencephalon. 



4. Metencephalon. 



5. Myelencephalon. 



These vesicles form the brain, their cavities becoming the ven- 

 tricles (Figs. 118, 120 and 17). The neuroblasts of the mantle 

 layer produce the neurones, whose cell-bodies and dendrites are 

 found in the cortex and ganglia, and whose medullated axones 

 form the white substance. 



Flexures (Fig. 118). The cephalic portion of the neural tube 

 is the seat of three flexures, two ventral and one dorsal, (i) The 

 mesencephalic flexure (ventral) begins very early and amounts to 

 nearly 180 degrees by the twenty-eighth day. It bends ventrally 

 the diencephalon until it almost touches the metencephalon. Thus 

 the inter-brain and pons are approximated and the mid-brain 

 almost concealed. (2) The cervical -flexure is also a ventral one. 

 It is located at the junction of the fifth vesicle with the spinal 

 cord, and corresponds to the bending of the head upon the body 

 of the embryo. This flexion begins about the twenty-first day. 

 By the end of the fourth week, it is completed and amounts to 

 90 degrees. (3) The dorsal flexure is beginning to form at the 

 same time (fourth week). It occurs between the fourth and fifth 

 brain vesicles, and is often called the metencephalic -flexure. It 

 reaches 180 degrees by the eighth week, when the dorsal part 

 of the metencephalon (the cerebellum) rests upon the medulla 

 oblongata. The convexity is formed by the pons, hence the 

 synonym, pontine flexure. The cervical and metencephalic flex- 

 ures almost entirely disappear, but the mesencephalic flexure is 

 permanent. 



