462 ELEMENTS OF BIOLOGY 



blood vessels which maintain connection between the mammalian 

 embryo and the maternal uterine wall. 



Undulating membrane. A delicate membrane associated with the gullet 

 in some of the Protozoa. 



Vaccine. An immunizing preparation consisting either of a culture of 

 weakened live bacteria or of killed bacteria. 



Valence. The number of bonds by which an atom attaches to other 

 atoms in forming molecules. 



Variation. Differences between members of the same species. Varia- 

 tions that are inheritable are now referred to as mutations; non- 

 inheritable variations produced by the effect of the environment 

 are referred to as acquired characters or modifications. 



Vegetal pole. Refers to that pole of an egg which is characterized by 

 being the region least active in development. The vegetal hemi- 

 sphere usually contains more yolk than the more active animal 

 hemisphere. 



Vestigial organs. Organs, either in the embryo or adult, without 

 apparent use and homologous with functional organs in lower 

 forms. 



Villi. Small folds of tissue. 



Viscera. The large interior organs of the animal body. 



Vitalism. The doctrine that regards life as due to some supernatural 

 activating principle. 



Vitamins. Substances occurring in small quantities in the food supply 

 necessary for continued maintenance of life. 



Viviparous (Adjective). Forms that give birth to well-developed young. 



Zoology. The science of animal life. 



Zygote. A fertilized ovum. 



