188 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



optic tracts are seen to terminate. The fibres grow in by the 

 optic stalk, decussate in the floor of the third ventricle between 

 the origins of the optic vesicles, and thus form the chiasma. 

 The optic fibres grow backwards on the surface of thala- 

 mencephalon (see Kg. 153) and on the optic thalamus to reach 

 the nerve centres which afterwards form the pulvinar, geniculate 



puluinar 

 thalamencephalon 



sup. corp. quad. 



olf. lobe 



int. genie, 

 ext. genie. 



yath of optic fibres 



yituit. 

 "optic stalk 

 "optic cup 



Fia. 153. — Diagram of the Foetal Brain at the end of the 2nd month, showing the 

 Position in which the Optic Tracts are developed. 



bodies and the superior corpora quadrigemina. In these centres 

 the optic fibres end. From some of the cells of these ganglia the 

 efferent fibres of the optic tracts are developed. 

 ~p("2) Tie basal ganglia. — The corpora quadrigemina. — Almost 

 in every structure the human embryonic condition resembles 

 the adult condition of lower vertebrates. A good example 

 is seen in the corpora quadrigemina, The human foetus 

 at the commencement of the third month (Fig. 153) shows 

 the corpora quadrigemina represented by a prominent thicken- 

 ing in the roof of the cavity of the mid-brain, which forms 

 subsequently the aqueduct of Sylvius. The thickening is 

 divided into lateral halves by a median sulcus, each half 

 being nearly as large as the cerebral vesicle of that period. In 

 Fig. 154 is shown the condition in an adult lizard; there is 



