GLOSSARY 511 



Synkaryon. The composite nucleus formed by the union of the nuclei of 

 two gametes. Male and female gametic nuclei united in the zygote. 

 See Zygote. 



Tapir. A large herbivorous Mammal, having short stout limbs and flexible 

 proboscis with the nostrils near the end. New World species are brown- 

 ish-black, those of the Old World are black and white. 



Taxonomy. The science of classification. 



Telophase. Final phase of mitosis during which the two daughter nuclei 

 are formed and cytoplasmic division is completed. See Prophase. 



Tetrad. Group of four chromosomes formed by a precocious division of 

 synaptic mates in the primary spermatocyte and primary oocyte. 



Thorax. Portion of the trunk in Mammals, containing esophagus, lungs, 

 and heart. The middle portion of the body in the Arthropoda; e.g., in all 

 Insects. In the Crayfish the head and thorax are fused to form the 

 cephalothorax. 



Thymus. A glandular structure in the pharyngeal region of Vertebrates. 

 Regresses during early life in Man. Function obscure. 



Thyroid Gland. An endocrine or ductless gland in the pharyngeal region 

 of Vertebrates. 



Tissue. An aggregation of similar cells for the performance of a certain 

 function. See Organ. 



Tissue Fluid. Essentially plasma and white blood cells that have passed 

 through the capillary walls to supply the milieu of the tissue cells. 

 Intercellular fluid. See Lymph. 



Tracheal System. Series of tubes that convey air throughout the tissues 

 of certain Arthropods; e.g., Insects. 



Trichocysts. Minute bodies, arranged in the outer part of the ectoplasm 

 of certain Infusoria (e.g., Paramecium), each of which upon proper 

 stimulation is transformed into a thread-like process protruding from 

 the cell surface. Apparently defensive structures. 



Trihyrrid. The progeny of parents differing in regard to three given 

 characters. 



Trilobites. Crustacea dominant during the early Paleozoic era. Ex- 

 tinct. 



Triploblastic. Composed of three primary germ layers: ectoderm, endo- 

 derm, and mesoderm, as in all animals above the Coelenterates. 



Trophozoite. Growing and feeding form developed from a Sporozoite; 

 e.g., in Monocystis and Malarial Parasite. 



Tropism. Element of behavior of the lower organisms. Orientation with 

 respect to an external stimulus; e.g., chemotropism, phototropism, etc. 



Tube Feet. Locomotor organs of Echinoderms; e.g., Starfish and Sea 

 Urchin. 



