914 TEXTBOOK OF ZOOLOGY 



Maxilla (maksira), the major bone of the upper jaw of vertebrates or the 

 accessory mouth part just back of the mandibles in many invertebrates. 



Medulla (me dul'a), posterior portion of the vertebrate brain; also the median 

 area of many organs. 



Medullary (med'uleri), pertaining to the medulla. 



Medullated (med'u lat ed), term used in reference to a nerve fiber which pos- 

 sesses a fatty or myelin sheath. 



Medusa (medu'sa), a free-swimming individual coelenterate, such as a jelly- 

 fish. 



Meiosis (mio'sis), the reduction division in maturation of germ cells. 



Menopause (men'opoz), natural cessation of menstruation in women between, 

 forty-five and fifty years of age. 



Meridional (me rid'i 6 nal), a condition in which planes extend from pole to 

 pole of a spherical body. 



Meroblastic (mer 6 blas'tik), having the type of egg structure in which cleavage 

 is only partial, owing to the accumulation of yolk in the egg. 



Mesencephalon (mesen sef'alon), the third region of the vertebrate brain, com- 

 monly called midbrain. 



Mesenchyme (mes'engkim) (Parenchj-ma), undifferentiated mesoderm com- 

 posed of large cells. 



Mesoderm (mes'odurm), the middle germ layer and its later derivatives. 



Mesoglea (mesogle'a), a jellylike substance found in Coelenterata between 

 the ectoderm and endoderm. 



Mesonephros (mes 6 nef'ros), the vertebrate kidney of forms from lamprey 

 to amphibians inclusive. 



MesorcMum (me sor'kium), the mesentery or membrane supporting a testis. 



Mesothelium (mes 6 the'lium), the mesodermic, membranous lining of the peri- 

 toneal cavity. 



Mesovarium (mes 6 va'rium), the mesentery in which the ovary is suspended. 



Metabolism (me tab'6 liz'm), the building up of living protoplasm and its con- 

 current oxidation. 



Metagenesis (met a jen'e sis), an alternation of sexual and asexual generation 

 in the life cycle of an organism. 



Metameres (met'amers), one of a series of similar parts that follow one another 

 in a vertebrate or articulate animal. 



Metamerism (me tam'er iz'm), serial symmetry or succession of segments. 



Metamorphosis (met a mor'fo sis), the transformation of one developmental 

 stage into another without intermediate steps. 



Metaphase (met'afaz), the phase of mitosis involving the longitudinal split- 

 ting of the chromosomes on the equatorial plate. 



Metazoa (met'azoa), animals whose bodies consist of few or many cells func- 

 tioning as a unit. 



Micronucleus (mi kro nu'kle us), the small nucleus of certain protozoans sup- 

 posed to control reproduction. 



Micropyle (mi'krdpil), the small opening in the egg where sperm enter in 

 certain forms of animals. 



Milt (milt), the light-colored spermatic fluid of male fish. 



Miracidium (ml ra sid'i iim), the early larval stages in the flukes. 



Mitochondria (mit 6 kon'dri a), small structures in the cytoplasm of animal 



cells; their significance is not entirely understood. 

 Mitosis (mito'sis), indirect cell division, involving the formation and splitting 

 of chromosomes and their equal distribution to daughter cells. 



Molt (molt), a complete or gradual shedding of the outer covering. 

 Monodelphia (mon 6 del'fi a), having a placenta. 

 Monoecious (mo ne'shiis). (See Hermaphroditic.) 



Monohybrid (mon 6 hi'brid), an offspring of parents which differ by only one 

 character. 



