458 ELEMENTS OF BIOLOGY 



Reflex act. A response to an outside stimulus that is effected without 

 the passage of the impulse through the brain. 



Regeneration. The restoration of a part that has been lost. 



Rejuvenescence. The restoration of a young condition. 



Rennin. An enzyme that curdles milk. 



Respiration. The cell process of oxygen intake, oxidation in the proto- 

 plasm, and carbon dioxide elimination from the cell. In complex 

 animals, breathing and the transportation of oxygen and carbon 

 dioxide by the blood are added processes. 



Respiratory quotient. The ratio between the carbon dioxide given off 

 and the oxygen taken in during respiration. 



Rickets. A metabolic disturbance characterized in the young of mam- 

 mals by under-development of bone and due to the absence of 

 Vitamin D from the diet. 



Salt. A compound formed when the hydrogen atoms of an acid are 

 replaced by some other type of atom. 



Sap vacuole. A cavity often present in plant cells, filled with cell sap. 



Saprophytic metabolism. The metabolism of parasitic plants and of 

 plants that subsist upon dead organic matter. 



Saprozooic metabolism. The metabolism of parasitic animals and of 

 animals that subsist on dead organic matter. 



ScoLEX. The head region of the tapeworm. 



Sebaceous glands. Oil glands in the integument of vertebrates. 



Secondary spermatocyte. A stage in spermatogenesis that results when 

 the primary spermatocyte divides, reducing the number of chromo- 

 somes to one-half the diploid number. 



Segregation. The separation of a character in pure form after it has 

 been associated in a previous generation with its alternative char- 

 acter. 



Self bred. Fertilization occurring by means of the union of egg and 

 sperm from the same individual. 



Seminiferous tubules. Tubules in the testes of vertebrates that are the 

 site of origin of the spermatozoa. 



Semi-permeable. A membrane is said to be semi-permeable if it allows 

 the passage of small molecules such as water and holds back large 

 molecules. 



Senescence. The process of becoming old. 



Septa. A partition; used to refer to the partitions that separate the 



