504 APPENDIX 



Metazoa. Multicellular animals with cells differentiated to form tissues. 

 Invertebrates and Vertebrates. See Protozoa. 



Micronucleus. The small ' germinal ' nucleus in Infusoria with dimorphic 

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

 have two, and P. woodruffi and P. polycaryum have several micronuclei. 

 See Macronucleus. 



Mitochondria. Bodies in the cytoplasm which apparently contribute to 

 specific chemical processes. 



Mitosis. The typical process of cell division. 



Modifications. See Acquired Characters. 



Monohybrid. The progeny of parents differing in regard to one given 

 character. 



Morphogenesis. The embryological development of the form and struc- 

 ture of an organism. 



Morphology. The science of the form of animals and plants. 



Mosaic Inheritance. Inheritance of a character in part from each parent 

 but without blending. 



Moult. To cast off the outside covering, such as the exoskeleton of 

 Arthropods. Ecdysis. 



Mutation. A heritable variation due to a change in the constitution of 

 the chromosome (gene) complex, independent of the usual processes 

 of segregation and crossing-over. Chromosomal aberrations and in- 

 trinsic gene changes. 



Myonemes. Contractile fibrils of certain Protozoa; e.g., Vorticella. 



Myotomes. Muscle segments in body wall of lower Vertebrates and em- 

 bryos of higher forms. 



Natural Selection. The processes occurring in nature which result in 

 the "survival of the fittest" individuals and the elimination of those 

 less adapted to the conditions imposed by their environment and mode 

 of life. Essence of the Darwinian theory of evolution. 



Nematocyst. A nettle-cell or stinging-cell of Coelenterates ; e.g.. 

 Hydra. 



Nephridiostome. Funnel-like opening of a nephridium into the coelom. 



Nephridium. A tubular excretory organ; e.g., in Earthworm. 



Nerve. Essentially a group or cable of parallel nerve fibers bound to- 

 gether. See Axon. 



Neural Tube. A tube derived from the ectoderm and forming the brain 

 and spinal cord in Vertebrates. 



Neurenteric Canal. Temporary passage between cavity of enteron 

 and neural tube in Vertebrate embryos; e.g., Frog. 



Neuron. A nerve cell, comprising cell body and cytoplasmic processes. 

 See Axon. 



