SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 219 



These expanded outgrowths are known as optic vesicles and the nar- 

 rowed connection of each to the brain is an optic stalk. The lateral 

 wall of the vesicle inverts into its cavity. The indented portion thus 

 forms the optic cup, the internal lining of which becomes the retina. 

 Slightly later, the crystalline lens is formed by a thickening of the 

 peripheral ectoderm which covers the optic cup. Along the ventral 

 side of the optic stalk is a furrow, the choroid fissure in which blood 



myclencephalon 

 visceral arch III 

 bulbo-conusarteriosus 



auditory vesicle 

 , / endolymphatic duct 



ganglion IX/ / ganglion VII-VIII 



atrium 



ventricle 



liver 

 prominence 



hyomandi bular cleft 

 mandibular arch 



ganglion V 



antenor 

 appendage bud i^ 



omphalo- 

 mesenteric 

 vein 



. meten— 

 cephalon 



mesen- 

 cephalon 



optic cup 



lens 



diencephalori 



epiphysis 

 telencephalon 



olfactory pit 



"S — omphalo- 

 mesenteric 

 artery. 



allantois 



Fig. 65. — View of right side of entire chick embryo of about four days of in- 

 cubation. Heart, brain, sensory organs, gill slits, and appendages are in the 

 process of development. (From Patten, Embryology of the Chick, P. Blakiston's 

 Son and Company.) 



vessels and the optic nerve will later lie. The choroid, sclera, ciliary 

 process, and muscles all develop from surrounding mesoderm, just a 

 little later. 



Olfactory organs begin as two areas of thickened ectoderm on the 

 head, anterior to the oral aperture. The plates of ectoderm sink to 

 form olfactory sacs which later open through to the mouth cavity, 

 or pharynx in higher vertebrates. The walls of the chamber thus 



