GLOSSARY 443 



MEGASPORE. The large spore which in heterosporous plants forms 

 a female gametophyte. 



MEGASPOROPHYLL. A modified leaf of a heterosporous sporophyte 

 which produces megaspores. A carpel. 



MERISTEM. Formative tissue with rapidly dividing cells, as in cam- 

 bium and growing points of plants. 



MESODERM. A primary tissue, or germ layer, of animals which 

 develops between ectoderm and endoderm. See Germ Layer. 



MESOGLOEA. The non-cellular layer between ectoderm and endo- 

 derm in Hydra and other Coelenterates. 



MESOPHYLL. Tissue of the leaf, between upper and lower epider- 

 mis, exclusive of the vascular bundles (veins). 



METABOLISM. The sum of the chemical processes in organisms, 

 involving the building up and breaking down of the living matter. 

 See Anabolism and Katabolism. 



METAGENESIS. Alternation of generations, as in Obelia. 



METAMERE. One of the series of similar parts, or segments, of the 

 body; e.g., in the Earthworm and Crayfish and, in highly modi- 

 fied form, throughout the Vertebrates. 



METAMORPHOSIS. A more or less abrupt transition from one devel- 

 opmental stage to another. E.g., in Insects. 



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. 



METAZOA. Multicellular animals. 



MICRONUCLEUS. The small 'germinal' nucleus in Infusoria with 

 dimorphic nuclei; e.g., Paramecium caudatum has one, and P. 

 aurelia and P. calkinsi have two micronuclei. See Macro- 

 nucleus. 



MICROSPORANGIUM. A sporangium which bears microspores; e.g., 

 pollen sacs in anther of stamen. 



MICROSPORE. The small spore, of heterosporous plants, which 

 forms a male gametophyte. A pollen grain. 



