282 GLOSSARY OF EMBRYOLOGICAL TERMS 



Heterozygous — condition in which zygote is composed of gametes bearing 

 allelomorphic genes. Opposed to homozygous. 



Hibernation — spending the cold (winter) period in a state of reduced ac- 

 tivity. 



Hindbrain — most posterior of the three original brain divisions. (Syn., 

 rhombencephalon. ) 



Hindgut — portion of amphibian embryonic gut just anterior to neurenteric 

 canal. Level of origin of rectum, cloaca, post-anal gut, and caudal 

 portions of urogenital systems. 



Histogenesis — development of tissues. 



Homoiothermal — pertaining to a condition in which temperature of body 

 of organism is under control of an internal mechanism; body tem- 

 perature regulated. Opposed to poikilothermal. 



Homology — similarity in structure based upon similar embryonic origin. 



Homoplastic — pertaining to a graft to an organism of same species, or even 

 to another position on the same individual. (Syn., autoplastic.) 



Homozygous — condition in which zygote is composed of gametes bearing 

 identical rather than allelomorphic genes. 



Horizontal — unsatisfactory term sometimes used synonymously with frontal, 

 longitudinal, and even sagittal plane or section. Actually means across 

 the lines of gravitational forces. 



Hormone — secretion of a ductless (endocrine) gland which can stimu- 

 late or inhibit activity of distant parts of biological system already 

 formed. 



Hyaloplasm — viscid liquid regarded as essential living protoplasm. 



Hybrid — successful cross between different species (e.g., horse and ass 

 give a mule, which is sterile). 



Hyoid Arch — mesodermal mass between hyomandibular and first branchial 

 clefts, or between first and second visceral pouches or clefts which give 

 rise to columella and parts of hyoid apparatus. (Syn., second visceral 

 arch. ) 



Hyomandibular — pertaining to pouch, cleft, or slit between mandibular and 

 hyoid arches. 



Hyperplasia — overgrowth; abnormal or unusual increase in elements com- 

 posing a part. 



Hypertrophy — increase in size due to increase in demands upon part con- 

 cerned. 



