GLOSSARY OF EMBRYOLOGICAL TERMS 309 



Visceral Mesoderm — See Splanchnic Mesoderm, Splanchnopleure. 



Visceral Plexus — aggregation of sympathetic neurons which control viscera, 

 having migrated posteriorly from tenth (vagus) cranial ganglia. 



Visceral Pouch — endodermal evagination of pharynx which, if it meets cor- 

 responding visceral furrow, often breaks through to form visceral cleft. 

 (Syn., pharyngeal pouch.) 



Vital Stain — localized staining of living embryonic areas with vital, non- 

 toxic dyes. 



Vitalism — a philosophical approach to biological phenomena which bases 

 its proof on present inability of scientists to explain all phenomena of 

 development. Idea that biological activities are directed by forces 

 neither physical nor chemical but which must be supra-scientific or 

 supernatural. Effective guidance in development by some non-material 

 agency. (See Mechanism.) 



Vitelline — pertains to yolk (e.g., vitelline vein brings blood from yolk; 

 vitelline membrane is that which covers yolked egg). 



Vitelline Artery — paired off shoots of dorsal aorta which take blood to 

 belly yolk of early embryo, later to become coeliac and mesenteric 

 arteries. 



Vitelline Membrane — delicate, outer, non-living egg covering derived while 

 egg is still within ovary, probably by joint action of egg and its 

 follicle cells; probably same membrane that is elevated as the fertiliza- 

 tion membrane after successful insemination. (Syn., fertilization mem- 

 brane.) 



Vitelline Substance — yolk. 



Vitelline Vein — paired veins, first to be formed in embryo, found in ventro- 

 lateral splanchnopleure, carrying nutritious blood from yolk region 

 to their junction with sinus venosus prior to the full development and 

 function of heart. 



Vitreous Humor — the rather viscous fluid of eye chamber posterior to lens, 

 formed by cells budded from retinal wall and from inner side of lens, 

 hence ectodermal and probably also mesenchymal in origin. (See 

 Aqueous Humor.) 



Wolffian Body — See Mesonephros. 



Wolffian Duct — See Mesonephric Duct, Urogenital Duct, Vasa Deferen- 

 tia. 



