GLOSSARY OF EMBRYOLOGICAL TERMS 289 



habitat from one that is aquatic to one that is terrestrial; change in 

 structure without retention of original form, as in change from 

 spermatid to spermatozoon. 



Metaphase — stage in mitosis when paired chromosomes are lined up on 

 equatorial plate midway between amphiasters, supported by mitotic 

 spindle, prior to any anaphase movement. 



Micromere — smaller of cells when there is a conspicuous difference in size, 

 characteristic of Annelids and Molluscs. 



Micropyle — aperture in egg covering through which spermatozoa may 

 enter; in such eggs the only possible point of insemination (e.g., many 

 fish eggs). 



Midbrain — See Mesencephalon. 



Midgut — that portion of archenteron which will give rise to intestines. 



Milieu — Term used to include all of the physico-chemical and biological 

 factors surrounding a living system (e.g., external or internal milieu). 



Mitochondria — small, permanent, cytoplasmic granules which stain with 

 Janus green B and Janus red; granules which have powers of growth 

 and division; probably lipoid. 



Mitosis — cytoplasmic division involving a nucleus and spindle apparatus. 



Mitotic Index — proportion in any tissue and at any specified time of the 

 dividing cells; percentage of actively dividing cells. 



Monospermy — fertilization accomplished by only one sperm. Opposed to 

 polyspermy. 



Monro, Foramina of — tubular connections between single third and paired 

 lateral ventricles of forebrain. 



Morphogenesis — all of the topogenetic processes which result in structure 

 formation; origin of characteristic structure (form) in an organ or in 

 an organism compounded of organs. 



Morphogenetic Movements — cell or cell area movements concerned with 

 formation of germ layers (e.g., during gastrulation) or of organ 

 primordia. 



Morula — spherical mass of cells, as yet without segmentation cavity. 



Mosaic — type of egg or development in which fate of all parts is fixed at 

 an early stage, possibly even at time of fertilization. Local injury or 

 excisions generally result in loss of specific organs in developing em- 

 bryo. Opposed to regulative development. 



Miillerian Duct — See Oviducts. 



