454 APPENDIX 



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 struc- 

 tures. 



TRIHYBRID. The progeny of parents differing in regard to three 

 given characters. 



TRILOBITES. Crustacea dominant during the early Paleozoic era. 

 Extinct. 



TURGOR. Outward pressure of the cell, largely due to the absorp- 

 tion of water, which distends the cell wall. The turgidity of the 

 individual cells results in the semi-rigid position of many plants. 

 Wilting results from a lowering of the turgidity of the cells. 



TYPHLOSOLE. A median dorsal invagination along the entire length 

 of the intestine of the Earthworm. Increases the area of the di- 

 gestive and absorptive surface. 



UMBILICAL CORD. A Mammalian structure, commonly known as 

 the navel cord, by which the embryo is attached to the placenta. 

 The blood vessels from the embryo to the placenta pass through 

 it. See Placenta. 



UNGUICULATE. Provided with claws. 



UNIFORMITARIAN DOCTRINE. An interpretation of the present con- 

 dition of the Earth on the assumption of similarity of factors at 

 work during past ages and to-day. 



UNIPARENTAL. Derived from a single progenitor; e.g., in asexual 

 reproduction. See Biparental. 



UNIT CHARACTERS. Characters which behave more or less as units 

 in heredity. 



UREA. Nitrogenous waste product of animal metabolism. Formed 

 as such in the liver, removed from the blood by the kidneys and 

 eliminated from the body chiefly in urine. 



URETER. A tube carrying urine from kidney to the cloaca or to the 

 urinary bladder. 



UROGENITAL. Relating to the urinary and reproductive systems. 



UTERUS. Lower portion of the oviduct (or oviducts) modified for 

 the retention of the eggs temporarily (Frog) or until develop- 

 ment has proceeded a considerable way and 'birth' occurs (Man). 



