THE BRAIN AND ITS DERIVATIVES 



165 



HYPOP 



OPTIC CHIASMA 

 OPTIC RECESS 



EPIPHYSIS 



CHOROID PLEXUS 



_VELUM 

 TRANSVERSUM 



LAMINA TERMINALS TQRUS JRANSVERSUS 



Fig. 97. The five primary divisions of the brain and some of their derivatives. 



tricks. The nerve fibers from the neuroblasts in the olfactory epithelium 

 grow back into the telencephalon and form the olfactory nerve. 



The diencephalon is bounded by a line running from the tuberculum 

 posterius to the epiphysis. The ventral wall forms the optic chiasma, where 

 the optic nerves cross as they enter the brain. The developing eye remains 

 attached to the diencephalon by means of the optic stalk. Nerve fibers origi- 

 nating from the neuroblasts in the retina grow back along this stalk and enter 

 the brain. The stalk becomes converted into supporting tissues for the nerves. 



The ventral wall of the diencephalon forms an evagination which be- 

 comes the nervous component of the pituitary body and is called the 

 infundibulum. The infundibulum fuses with an outgrowth of the ectoderm 

 of the oral cavity. This ectodermal outgrowth is often called the hypophysis, 

 but actually it is only one of the two components of the adult hypophysis. 

 These two components of the adult pituitary body have vastly different 

 functions. The ectodermal element, which forms the anterior lobe of the 

 pituitary, secretes several hormones, such as the gonadotropic and growth 



