GLOSSARY 171 



maturation. Process of ripening; especially a stage in the 

 formation of spermatozoa and ova. 



maxilla. One of the mouth parts of arthropods; in verte- 

 brates the jaw bone. 



maxilliped. Foot jaws. Thoracic appendages of the cray- 

 fish modified as mouth parts. 



medusa. The free swimming, sexual individual in the life 

 history of hydroids. 



mesentery. A thin fold of tissue which holds the intestine 

 in place against the body wall. 



mesodenn. The middle one of the three germ layers formed 

 in the development of most embryos. 



mesothorax. The middle segment of the thorax of insects. 



metamere. One of the serially arranged body segments or 

 somites in animals such as Annelida and Arthropoda. 



metamorphosis. The striking changes in form undergone 

 by certain animals in the course of development after 

 the commencement of a free existence. For example, 

 the change from a caterpillar through a pupa into a 

 butterfly, the change of a tadpole into a frog. 



metaphase. The stage in mitosis during which the chromo- 

 somes are split. 



metathorax. The posterior somite of the thorax of insects. 



micronucleus. The small nucleus or nuclei found in certain 

 protozoa, always accompanied by a macronucleus. 



mitosis. Indirect division during which the nucleus under- 

 goes complicated changes, and the chromatin becomes 

 divided into equal halves. 



morphology. The science of the form and structure of 

 animals and plants. 



mucosa. Mucous membrane. An epithelial covering which 

 is kept moist by secretions of mucus, as in the stomach. 



