GLOSSARY OF TECHNICAL TERMS xxxix 



which an embryo or foetus undergoes development ; the womb 



in mammals. 

 VACUOLB (Lat. vacuus, empty), a space in the protoplasm of a cell, 



filled with gas or liquid. 

 YAS DEFERENS (Lat., a vessel which carries down), a duct through 



which spermatozoa pass to the outside of the body. 

 VASCULAR BUNDLE (Lat. vasculum, a small vessel), a bundle of 



vessels, &c., through which sap is distributed in higher 



plants. 

 VERTEBRA (Lat., joint, vertebra), one of the segments of the 



backbone. 



VERTEBRATE, having a backbone. 

 VESTIGIAL (Lat. vestigium, trace, vestige), reduced almost to the 



point of disappearance. 

 VITALISM (Lat. vita, life), the doctrine^jg^ch attributes the 



phenomena of life to a special vitaTforce.* 

 VITELLINE MEMBRANE (Lat. vitellus, yolk), a delicate membrane 



which encloses an ovum. 



XYLEM (Gr. vXov, wood), the woody part of a vascular bundle. 

 ZO^EA (Gr. C*? ^ animal), a larval stage of crabs and other 



Crustacea. 

 ZONA RADIATA (Lat., radiate zone), a membrane which encloses the 



ovum in mammals, outside the vitelline membrane. 

 ZOOID (Gr. 6jW, animal ; 1609, form), an individual member 



of a colony (e.g., in Obelia). 

 ZOOPHYTE (Gr. fwor, animal ; fyvrov, plant), an old term applied to 



certain plant-like invertebrates (e.g., Obelia). 

 ZOOSPORE (Gr. fooo's, alive ; a-nopd, seed), an actively swimming 



spore. 



ZYGOSIS (Gr. Qyavis, joining together), another term for con- 

 jugation or the sexual union of gametes. 

 ZYGOSPORE (Gr. C^yo'co, I join together ; a-iropd, seed), a zygote 



formed by the conjugation of two similar gametes (e.g., in 



Spirogyra). 



ZYGOTE (Gr. (bywros, joined together), a cell produced by the con- 

 jugation of two gametes and capable of developing into a new 



individual. 



