GLOSSARY 



487 



METABOLISM, ANIMAL - metabolism which 

 brings about or is associated with the dif- 

 ferentiation in the animal (ectodermal) 

 direction (e. g. , sea urchin eggs in sulphate 

 ions) characterized by increased oxygen 

 consumption. Can be checked by lithium. 

 (See animalization. ) 



METABOLISM, VEGETAL - metabolism which 

 brings about or is associated with the dif- 

 ferentiation in the vegetal (endodermal) 

 direction (e. g. , sea urchin eggs in lithium 

 chloride) characterized by a breakdown of 

 proteins and checked by an absence of sul- 

 fate ions. (See vegetativisation. ) 



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 eye- 

 lids and limbs; changes in structure cor- 

 related with changes in habitat from one 

 that is aquatic to one that is terrestrial; 

 change in structure without retention of 

 original form, as in the change from 

 spermatid to spermatozoon. 



METAMORPHOSIS, ANURAN - loss of tail, 

 larval mouth, and gills; reduction in the 

 gut; development of limbs. Period ends 

 with the appearance of the tympanum. 



METAMORPHOSIS, URODELE - period of gill 



reduction, shedding of skin and the develop- 

 ment of eyelids. 



METAPLASIA - permanent and irreversible 



change in both type and character of cells; 

 transformation of potencies of an embry- 

 onic tissue into several directions, gen- 

 erally an indication of a pathological condi- 

 tion (e. g. , bone formation in the lung). It 

 is thought that some differentiated tissue 

 nriay become undifferentiated and then un- 

 dergo a new differentiation in a different 

 direction. 



METATHETELY - the appearance of early em- 

 bryonic structures at a stage later than 

 normal (e. g. , the retention 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. 



MICROGNATHUS - retarding of lower jaw in the 

 new born. 



MICROMERE - smaller of the cells when there 

 is variation in the size of blastomeres. 



MICROMETRY - 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 micropyles. 



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 instruments of micro- 

 scopic 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 

 melieu). 



MITOCHONDRIA - small, permanent cytoplasmic 

 granules which stain with Janus Green B, 

 Janus Red, Janus Blue, Janus Black 1, Rho- 

 damin B, Dietheylsafranin, 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 enzymes. 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 proliferation. Such rays come 

 from disintegrating, dead tissues of regen- 

 erating tails (e.g., axolotls - Blacher, 1930). 



MITOTIC INDEX - the number of cells, in each 

 thousand, which are in active mitosis at any 

 one time and place in an organism (Minot, 

 1908); 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 on- 

 togeny). 



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 - Waddington). 



MOLTING - periodic shedding of the upper, cor- 

 nified epidermis, common among amphibia 

 and reptiles, and possibly associated with 

 breeding activity. 



MONOSPERMY - fertilization accomplished by 

 only one sperm. Opposed to polyspermy. 



MONSTER, AUTOSITE-PARASITE - double em- 

 bryos with great size discrepancy so that 

 the smaller one bears a parasitic relation- 

 ship to the larger; variously produced. 



MONSTER, DICEPHALUS - double-headed ab- 

 normality, produced by any means. 



