GLOSSARY 



HEAD. The front end of an animal, or part, if it is (iistint^niishrd from the nsi in 



any way. 

 HEART. A pulsating; ort^an of the hlood-vascular system; a blood vessel modilied lor 



propelling; the blood. 

 HEMOCOEL. A body cavity, which may be a pseudocoel or may develop in part from 

 a true coelom, containing a fluid that may be called blood or lymph and func- 

 tioning as part of the circulatory system. 

 HEMOGLOBIN. The pigment of I he blood with which oxvgen combines to form 



o.xyhemoglobin. 

 HEPATIC. Pertaining to the liver. 



HERBIVOROUS. Feeding upon jilants. CJ. CARNIVOROUS and OMNIVOROUS. 

 HEREDITARY DETERMINERS. See GENES. 

 HEREDITARY UNITS. See GENES. 

 HEREDITY. Ihe resemblance of successive generations of individuals, conditioned 



1)\ the transmission of genes during reproduction. 

 HERITABLE VARIATION. A modification in either structure or function that recurs 



in successive generations; a variation conditioned bv changes in a gene or in a 



s^ene complex. Cf. MUTATION, CHROMOSOMAL ABERRATION, and FLUCTUATION. 

 HERMAPHRODITIC. Possessing both male and female gonads; monoecious. 

 HETEROZYGOTE. A zvgote formed by the union of two gametes that differ with 



respect to particular genes. CJ. HOMOZYGOTE. 

 HETEROZYGOUS. Developing from a heterozygote. CJ. HOMOZYGOUS. 

 HIBERNATION. A cessation of activity by an animal during cold weather. 

 HISTOGENESIS. The difTerentiation of tissue cells. CJ. GAMETOGENESIS. 

 HISTOLOGY. The science of the structure of animals as rexealed by the microscope; 



the stud\ of cells as they are grouped to form tissues and organs. (,'/. ANATOMY. 

 HOLOPHYTIC. .\ form of nutrition characteristic of green plants and of chloroph\ 11- 



bearing plant-like protozoans. It involves the photosynthetic formation of 



carbohydrates from carbon dioxide and water. CJ. SAPROZOIC and HOLOZOIC. 

 HOLOZOIC. A form of nutrition in which foods are ingested in .solid or particulate 



form (J. HOLOPHYTIC and SAPROZOIC. 

 HOMOLOGOUS CHROMOSOMES. A pair of chromosomes similar in size and shape, one 



of which is contributed by each of the gametes that unite to form the zygote. 

 HOMOLOGY {adj. HOMOLOGOUS). Correspondence in fundamental structure or in 



embryonic and presumably phylogenetic origin. CJ. ANALOGY. 

 HOMOiTYGOTE. A zygote formed by the union of two gametes that are alike with 



res[)ect to particular genes. CJ. HETEROZYGOTE. 

 HOMOZYGOUS. Developing hom a homozygote. CJ. HETEROZYGOUS. 

 HORMONE. .See ENDOCRINE. 

 HOST. An organism that nourishes a parasite. 

 HYBRID. An individual whose parents difTered with respect to one or more characters. 



(J. HYBRIDIZATION. 

 HYBRIDIZATION. The breeding of individuals that differ with respect to one or more 



characters. See EXPERIMENTAL BREEDING. 

 HYPER-. Combining form, meaning ''over," either in position or degree. Cf. HYPO- 



and SUPER-. 

 HYPO-. Combining form, meaning "under," either in position or degree. Cf. HYPER-. 

 HYPOTHESIS. A tentative correlation or explanation of observed facts which is 



u.suallv stated as an aid to further study of related facts; if supported bv addi- 

 tional data, a hypothesis mav become a theory. 



IDENTICAL TWINS. Two individuals, always of the same sex, which arise during 



development from a single zygote. CJ. FRATERNAL TWINS. 

 IMAGO. Adult stage of an insect. 



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