GLOSSARY 503 



Lymph. Essentially excess tissue fluid passing through vessels on its way 

 back to the blood vascular system. See Tissue Fluid. 



Macronucleus. The large 'somatic' nucleus in Infusoria with dimorphic 

 nuclei; e.g., in Paramecium. See Micronucleus. 



Madreporic Plate. A small perforated plate on the aboral surface of 

 certain Echinoderms (e.g., the Starfish) that allows the passage of water 

 to the water- vascular system. 



Maltose. A double sugar derived from starch by hydrolysis during diges- 

 tion. 



Mandibles. Jaws. 



Mantle. Layer of tissue that secretes the shell in Molluscs. 



Maturation. Final stages in the formation of the germ cells, involving 

 chromosome reduction (meiosis). 



Maxillipeds. The three anterior pairs of appendages of the thorax of the 

 Crayfish. 



Mechanism. The doctrine that the phenomena of life are interpretable 

 in terms of the laws of matter and energy which hold in the realm of the 

 non-U ving. See Vitalism. 



Medusa. Sexual, gonad-bearing generation of hydra-like animals, the 

 Hydrozoa, and also the Scyphozoa. 



Meiosis. See Reduction. 



Mendel's Laws. See Segregation and Independent Assortment. 



Mesentery. Fold of the peritoneal lining of the body cavity, suspend- 

 ing the alimentary canal. Also, radial partitions in the enteric cavity; 

 e.g., of Metridium. 



Mesoderm. A primary tissue, or germ layer, of animals, which develops 

 between the ectoderm and endoderm. See Germ Layer. 



Mesogloea. The non-cellular layer between the two primary tissue 

 layers of Coelenterates. 



Mesorchium. Mesentery-like membrane supporting the testes. 



Metabolism. The sum of the physico-chemical processes in organisms, 

 involving the building up, maintenance, and breaking down of the 

 living matter and its constituents. See Anabolism and Katab- 

 olism. 



Metamorphosis. A more or less abrupt transition from one developmen- 

 tal stage to another; e.g., transformation of larva into adult during the 

 life history of a Butterfly or Frog. 



Metaphase. Climax of mitosis involving the separation of the halves of 

 the longitudinally split chromosomes arranged in the equatorial plate. 

 See Anaphase. 



Metaphyta. Multicellular plants. 



Metaplasm. Lifeless inclusions in cytoplasm; e.g., yolk granules, etc. 



