GLOSSARY 455 



UTRICULUS. The posterior sac of the labyrinth of the ear into 

 which the semicircular canals open. 



VASCULAR BUNDLE. Composite of xylem, cambium, phloem, and 

 bundle sheath. Except for the cambium, essentially a system 

 of tubes for conducting water and food. A fibre-vascular 

 bundle. See Stele. 



VASO-MOTOR NERVES. Nerves which regulate the calibre of small 

 arteries by bringing about relaxation or contraction of the 

 muscular layer of their walls. 



VERMIFORM APPENDIX. Blind outpocketing of the large intestine 

 near its origin from the small intestine. Vestigial end of the 

 caecum. Found only in Apes and Man. 



VERTEBRA. One of the series of elements forming the backbone, 

 or vertebral column. 



VERTEBRATE. An animal with a backbone, or vertebral column. 



VITALISM. The doctrine which attributes at least some of the phe- 

 nomena of life to an interplay of matter and energy which tran- 

 scends the so-called laws* operable in the inorganic world. See 

 Mechanism. 



VITAMINES. Indispensable accessory food substances whose im- 

 portance has but recently been realized. Chemical composition 

 is as yet practically unknown. 



WORKING HYPOTHESIS. A basic assumption to guide the study of 

 a subject, and to be proved or disproved by facts accumulated. 



X CHROMOSOME. The 'accessory' or 'sex-chromosome.' 



XYLEM. The inner woody part of a vascular bundle. 



YEAST. A group of unicellular colorless plants (Fungi) which are 

 chiefly responsible for alcoholic fermentation. 



YOLK. Food material stored within the cytoplasm of an egg. See 

 Metaplasm. 



ZOOGEOGRAPHY. The science of the geographical distribution of 

 animals. 



ZYGOTE. The composite cell formed by the union of male and 

 female gametes. See Synkaryon. 



