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 CiiAnACTER. 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 deiinite parts of an egg or embryo. 



Lol'-i-go. The squid, a genus of cephalopod mollusks. 



Mar-sxt'-pi-als. A primitive group of mammals, including op- 

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



Mat-u-ra'-tion. 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-deem. 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. 



