GLOSSARY 513 



Vitalism. The doctrine which attributes at least some of the phenomena 

 of life to an interplay of matter and energy which transcends the so- 

 called laws operable in the inorganic world. See Mechanism. 



Vitamins. Indispensable accessory food substances whose importance 

 has but recently been realized. Chemical composition of most of them 

 is unknown. 



Viviparous. Producing young that have developed to a relatively ad- 

 vanced stage in the uterus; e.g., most Mammals. See Oviparous. 



Working Hypothesis. A basic assumption to guide the study of a sub- 

 ject, and to be proved or disproved by facts accumulated. 



X Chromosome. The so-called sex chromosome. 



Y Chromosome. The synaptic mate of the X chromosome when present. 

 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 Meta- 



plasm. 

 Yolk Sac. When a great quantity of yolk is present in an egg, the endo- 



derm gradually grows over it to form the yolk sac which is usually of 



enormous size in comparison with the embryo proper. 



Zoogeography. The science of the geographical distribution of animals. 

 Zygote. The composite cell formed by the union of male and female 

 gametes. Fertilized egg. See Synkaryon. 



