GLOSSARY 1*71 



MESENCHyWE - the form of embryonic meaoderm or meeotlast in which migrating cells unite secondarily to form 

 a syncitium or netvork having nuclei In thickened nodes between Intercellular spaces filled with fluid. 

 Often derived from mesothelium. 



MESIAL - Syn., median, medial, middle. 



MESODEHM - primary germ layer which arises from the marginal zone to take up Its assigned position between 

 the outer ectoderm and the inner endoderm. 



MESOMERE - cells of intermediate size when there are cells of various sizes (macromeres and micromeres be- 

 ing the largest and the smallest, respectively). Also used as synonym for intermediate cell mass 

 which gives rise to the nephrlc system. 



METABOLISM - the sum total of chemical changes occurring in the life of an organism. 



METABOLISM, ANIMAL - metabolism which brings about or is associated with the differentiation in the animal 

 (ectodermal) direction (e.g., sea urchin eggs in sulphate ions) characterised by increased oxygen 

 consumption. Can be checked by lithium. (See animallzatlon. ) 



METABOLISM, VEGETAL - metabolism which brings about or is associated with the differentiation In the 



vegetal (endodermal) direction (e.g., sea urchin eggs In lithium chloride) characterized ty a break- 

 down of proteins and checked by an absence of sulfate ions. (See vegetativisatlon. ) 



METAMORPHOSIS - the end of the larval period of amphibia when growth is temporarily suspended. The change 

 is from the larval (aquatic) to the adult (terrestrial) form. There Is autolysis and resorption of 

 old tissues and organs such as gills, and the development of new structures such as eyelids and limbs; 

 changes In structure correlated with changes in habitat from one that is aquatic to one that is 

 terrestrial; change in structiu-e without retention of original form, as in the change from spermatid 

 to spermatozoon. 



METAMORPHOSIS, ANUEAN - loss of tall, larval mouth, and gills; reduction In the gut; developnent of limbs. 

 Period ends with the appearance of the tympanum. 



METAMORPHOSIS, URODELE - period of gill reduction, shedding of skin and the development of eyelids. 



METAPLASIA - permanent and Irreversible change in both type and character of cells; transformation of 



potencies of an embryonic tissue Into several directions, generally an Indication of a pathological 

 condition (e.g., bone formation In the lung). It Is thought that some differentiated tissue may 

 become undifferentiated and then undergo a new differentiation In a different direction. 



METATHETELY - the appearance of early embryonic structures at a stage later than normal (e.g., the reten- 

 tion of larval organs by Insect pupae). Opposed to prothetely. 



MICROCEPHALUS - small or pin-headed; a condition due to the arrested development of the cranium and the 

 brain, accompanied by reduced mentality. 



MICHOGNATHUS - retarding of lower Jaw In the new bom. 



MICEQMERE - smaller of the cells when there Is variation In the size of blastomeres. 



MICSCMBTRY - measurement of a microscopic object, using an ocular micrometer. 



MICROPHTHALMIA - eyes that are too small, often due to undersized lenses (Harrison, 1929). 



MICROPYLE - an aperture In the egg covering (e.g., fish eggs) through which spermatozoa may enter. 

 Generally the only possible point of fertilization in eggs bearing mlcropyles. 



MICROSOMIA - dwarfism, reduced skeleton, due possibly to disturbed function of the pituitary and thyroid 

 glands. 



MICROSTOMUS - small mouth; excessive closure of the mouth. 



MICROSURGERY - procedures described by Spemann, Chambers, Harrison and others where steel and glass In- 

 struments of microscopic dimensions are used to operate on small embryos. 



MILIEU - term used to Include all of the physico-chemical and biological factors surrounding a living 

 system (e.g., external or Internal melleu). 



MITOCHONDRIA - small, permanent cytoplasmic granules which stain with Janus Green B, Janus Red, Janus 



Blue, Janus Black 1, Ehodamln B, Dletheylsafranin, dilute methylene blue, and which have powers of 

 growth and division and are probably lipoid In nature, and may contain proteins, nucleic acids, and 

 even ertzymes. Syn., plastens. 



MITOGENETIC RAYS - rays of short wave-length emanating from a growing point (e.g., onion root tip - 



Gurwitsch, 1926) which rays excite cell division when they encounter tissues capable of prolifera- 

 tion. Such rays come from disintegrating, dead tissues of regenerating tails (e.g., axolotls - 

 Blacher, 1950). 



MITOTIC INDEX - the number of cells. In each thousand, which are In active mitosis at any one time and 

 place In an organism (Minot, I908); the percentage of actively dividing cells. Often considered as 

 a measure of growth activity. 



MODULATION - physiological fluctuation of a cell in response to environmental conditions, indicating 

 latitude of cell adaptation; cellular changes reversible without residue (Weiss, 1939); temporary 

 reactions of cells to new environmental conditions without loss of original potential functions 

 (e.g., reversible histological differentiation at the end of ontogeny). 



MODULATOR - specific Inducing substance which goes beyond basic evocation and will Induce a specific 

 kind of tissue characteristic of a definite region (e.g., neural tube of mid-body level - 

 Waddlngton) . 



MOLTING - periodic shedding of the upper, comlfled epidermis, common among amphibia and reptiles, and 

 possibly associated with breeding activity. 



MONOSPERMY - fertilization accomplished ty only one sperm. Opposed to polyspermy. 



MONSTER, AUTOSITE- PARASITE - double embryos with great size discrepancy so that the smaller one bears a 

 parasitic relationship to the larger; variously produced. 



MONSTER, DICEPHALUS - double-headed abnormality, produced by any means. 



MONSTER, ISCHIOPAGUS - double embryos, widely separated except at the tail; produced by any means. 



