GLOSSARY 451 



Isotonic A solution the osmotic pressure of which is exactly equal to 



that of its surroundings. 

 Kinetic energy. Energy in action. 



Lacteals. Fine vessels of the lymphatic system that occur in great pro- 

 fusion in the walls of the intestine. 

 Leucocytes. White blood cells. 

 Life. A collective word used to include all the phenomena that are 



characteristic of matter in the living state. 

 Lipase. Fat-splitting enzymes. 

 Macronucleus. The larger member of two nuclei that occur in some 



types of Protozoa. 

 Macrospore. a cell developed in the base of a flower which subse- 

 quently develops into the egg-producing gametophyte generation. 

 Malpighian tubules. Fine tubules that constitute the excretory organs 



and empty into the intestine of insects. 

 Mechanism. The doctrine that regards life processes as manifestations of 



physical and chemical principles. 

 Mediastinum. The partition separating the right and left pleural cavities 



in mammals; it is traversed by the trachea, the cesophagus, main 



blood channels, and nerves. 

 Membrane. A partition between two substances. Living membranes are 



of three types: non-cellular basement membranes that separate 



layers of cells; the cell boundary; membranes composed of cell 



layers, for example, the lining of the intestine. 

 Membrane bones. See Dermal bones. 

 Mesoderm. The intermediate layer of cells that develops between the en- 



doderm and the ectoderm. 

 Mesolecithal (Adjective). Eggs that are intermediate between isolec- 



ithal and teleolecithal as regards quantity and distribution of yolk. 

 Mesonephros. The excretory organ of most fishes and all Amphibia. It 



also develops in the embryos of mammals but does not function 



in some embryos. It becomes converted into important parts of the 



male reproductive system of the mammal. 

 Metabolism. A collective word used to include all the chemical activities 



of living protoplasm. 

 Metagenesis. A life cycle that includes sexual and asexual generations 



in regularly occurring sequence. 

 Metamerism. A type of animal structure which is characterized by the 



