ANATOMY OF A 17.8 MM. HUMAN EMBRYO 51 



evagination of the optic vesicles it is somewhat questionable 

 whether this belongs primarily with the tel- or di-encephalon. 



Diencephalon. The bomidary between the diencephalon and 

 the mesencephalon (Mesen.) is a slight constriction extending 

 nearly transversely across the brain toward the tuberculmn 

 posterius {Tub. p.). An ill defined furrow, sulcus limitans, ex- 

 tends cephalo-caudad along the internal surface of the dienceph- 

 alon separating the dorsal zone above from the ventral zone or 

 hypothalamus below. The roof of the diencephalon is thin; near 

 its caudal limit there is a slight evagination which represents the 

 first appearance of the epiphysis (Corp.pin.). 



The ventral part of the dorsal zone caudad of the interventricu- 

 lar foramen is thickened, forming on the medial surface a low 

 ridge, the developing optic thalamus. Dorsal to the thicken- 

 ing, the internal surface of each dorsal zone presents a prominent 

 concavity. The lateral wall of the hypothalamus is thickened. 

 Caudad of the recessus postopticus (R.po.-op.) the cavity of the 

 diencephalon extends into a small median evagination from the 

 floor-plate. The evagination is the anlage of the infundibular 

 gland {Gl.inf.). The thickened knob-like termination of the 

 gland is embraced by the forked distal end of the hypophysis 

 (Hyp.). It becomes the neural lobe of the adult pituitary. 



Mesencephalon. The conventional boundary between the 

 mesencephalon (Mesen.) and metencephalon (Meten.) is the con- 

 stricted portion or isthmus (Isth.). As already stated a slight 

 constriction extending nearly transversely across the brain 

 toward the tuberculum posterius {Tuh.p.) divides it from the 

 diencephalon. The roof -plate is comparatively thin, and bears 

 no trace of the longitudinal ridge which has been described as 

 occurring later. The floor-plate is considerably thicker than the 

 roof-plate. The dorsal and ventral zones are distinct. The 

 cavity of the mesencephalon (cerebral aqueduct) is more expanded 

 in the caudal two-thirds of the mesencephalon, a large oval con- 

 cavity appearing on the internal surface of each dorsal zone. 

 The ventral zones are considerably thickened so that in the region 

 of the oculo-motor nerve they project ventrally below the floor- 

 plate. Externally the surface of the mesencephalon ds smooth 



