34 



THE NERVOUS SYSTEM 



month. The most striking feature of the brain at this stage is the great size 

 attained by the cerebral hemispheres. 



The Diencephalon. The three principal divisions of the diencephalon 

 the thalamus, epithalamus, and hypothalamus faintly indicated in an embryo 



Diencephalon, 

 Chorioid plexus 



Corpus striatum 

 Telencephalon / 



Thalamus 



I Pineal body (epithalamus) 

 Cerebral peduncle 

 Cerebral aqueduct 

 " Mesencephalon 



'-Isthmus 

 '*- Cerebellum 

 ~ Metencephalon 

 Rhomboid fossa 

 Myelencephalon 



: Optic Hypo- 

 j chiasma, physis Medulla 



Lamina terminalis / "Hypothalamus blon z ata 

 Rhinencephalon 



' Spinal cord 

 Central canal 



Fig. 17. The brain of a fetus of the third month in median sagittal section. (His, Sobotta.) 



of 13.6 mm., are well denned by the third month (Fig. 17). In transverse 

 sections this division of the embryonic brain is seen to be composed of a pair of 

 plates on either side, which with a roof and floor form the walls of the ventricle 



Roof plate (with chorioid plexus) 



Alar plate or Thalamus 



Sulcus limitans 

 Basal plate or Hypothalamus 



"Mammillary recess 



Fig. 18. Transverse section through the diencephalon of a 13.8 mm. embryo. (His, Prentiss- 



Arey.) 



(Fig. 18). The dorsal lamina is known as the alar plate, the ventral as the basal 

 plate. On either side these meet at an angle, forming the sulcus limitans. These 

 laminae and the sulcus limitans between them can be traced back through the 



