508 GLOSSARY 



IDENTICAL TWINS. Twins which have come from a single egg 

 and which show identical hereditary characters. 



ID'-I-O-PLASM. The germ-plasm or inheritance material. 



INDUCTION. A modification of the first filial generation caused 

 by the action of environment on the germ cells of the 

 parental generation. 



INHERITED CHARACTER. A character the differential cause of 

 which is in the germ. 



INSTINCTS. Complex reflexes involving nerve centers. 



INVERSE SYMMETRY. Having the right half of one asymmetri- 

 cal individual equivalent to the left half of another. 



IRRITABILITY. Capacity of receiving and responding to stimuli. 



LA-MARCK'-ISM. The doctrine that evolution takes place 

 through the inheritance of acquired characters. 



LOCALIZATION. The gathering together of particular sub- 

 stances in definite parts of an egg or embryo. 



LOL'-I-GO. The squid, a genus of cephalopod mollusks. 



MAR-SU'-PI-ALS. A primitive group of mammals, including op- 

 posums and kangaroos, which carry the young in a pouch. 



MAT-u-RA'-TfoN. The final stages in the formation of sex cells, 

 characterized by two peculiar cell divisions. 



ME-RIS'-TIC VARIATION. Variation in the number of parts. 



MES'-EN-CHYME. Loosely scattered cells of the mesoderm. 



MES'-O-DERM. A layer or group of embryonic cells lying be- 

 tween ectoderm and endoderm. 



ME-TAB'-O-LISM. Transformations of matter and energy within 

 a living thing. 



MI'-CRO-PYLE. The minute opening in an egg membrane 

 through which the spermatozoon enters. 



MI-TO'-SIS. Indirect nuclear division in which the nucleus is 

 transformed into a spindle and chromosomes; the latter 

 split and the halves move to the poles of the spindle 

 where they form the daughter nuclei. 



MON-O-HY'-BRID. The offspring of parents differing in one 

 character. 



MON'-O-TREMES. The lowest group of mammals, including the 

 duck-bill and the spiny anteater. 



