678 Glossary 



M 



Maorogamete — the larger or female gamete. 

 Malpighian layer — the actively dividing layer of the epidermis. 

 Mammary glands — the glands of mammals that produce milk for the nourish- 

 ment of the young. 

 Marrow — vascular soft tissue found in the cavities of most bones. 

 Medial — toward the middle of the body. 



Median plane — plane dividing the body into right and left halves. 

 Mediastinum — the space in the thoracic cavity between the lungs that contains 



the heart, thymus, and large blood vessels. 

 Medulla — the inner portion of any organ; part of the brain. 

 Medusa — jellyfish; the sexual stage of some coelenterates. 

 Meiosis — the process that results in the production of gametes with a haploid 



number of chromosomes. 

 Membrane bone — a bone which is formed in connective tissue. 

 Meninges — the membranes which envelop the brain and spinal cord. 

 Mesoderm — the middle embryonic germ layer from which the bulk of the body 



organs develop. 

 Metabolism — the sum of the processes involved in the building up and tearing 



down of protoplasm. 

 Metagenesis — alternation of sexual and asexual generations. 

 Metamerism — serial segmentation. 



Metazoa — animals whose bodies consist of more than a single cell. 

 Metanephridial system — an excretory system in which each unit consists of a 



tubule with a ciliated funnel which usually opens into the body cavity. 

 Microgamete — the smaller or male gamete. 

 Mitosis — a process of cell division in which the resulting cells have the same 



number and kind of chromosomes as the original. 

 Molar — a kind of tooth adapted for grinding. 



Mollusca — a phylum of animals that contains the clams, snails, and octopuses. 

 Molt — periodic shedding. 

 Monoecious — having both male and feinale reproductive organs in the same 



individual. 

 Monohybrid — a genetic cross involving a single pair of characters. 

 Morula — a stage of early embryology in which there is a mass of cells formed 



by cleavage. 

 Motor — a type of nerve or nerve fiber that carries impulses from the central 



nervous system or a ganglion to a muscle or gland. 

 Mucosa — the inner lining of the intestine. 



Muscle — an organ that produces movement ; the tissue composing the organ. 

 Mutation — a sudden change in a gene resulting in a phenotypic modification. 

 Myelin — a sheath of fatty tissue surrounding some nerve fibers. 

 Myofibril — longitudinal fibril found within a muscle cell. 

 Myotome — muscle segments. 



