330 



Tuberculum posterius, 165 

 Twinning, 



of fish egg, 36 



theory of, 37, 38 



Umbilical artery (see also Allantoic ar- 

 tery) , 



of chick, 174, 178 

 Undifferentiated cells, in tail bud, 222 

 Unequal cleavage, 1 1 5 

 Urea, 



in chick, 196, 198 



in mammals, 258, 277 

 Ureter, in chick, 196, 197 

 Uric acid, 



in chick, 196, 198 



in mammals, 258, 277 

 Uterine cycle, 259 

 Uterine milk, 276 

 Uterus, 



contraction of muscles of, 262 



implantation of blastocyst in, 260, 261 



menstruation in, 259 



mucosa of, 268 



muscle of, 268 



postovulatory period of, 259 



preovulatory period of, 259 



Vagus nerve, 163, 164 



Vas deferens, in chick, 198 



Vegetal pole, 27 



Ventral aorta, in frog embryo, 136 



Ventral horn, 162 



Ventricle, 



of brain, 165, 166 



in chick, 



48-hr., 216, 220 

 72-hr., 223, 236 



of heart, 216, 220,223,236 

 Vernix caseosa, 287 

 Vertebrae, 



formation of, 195 



origin of, 163 

 Viability of sperm and egg, 51, 53 



INDEX 



Villus, chorionic, 268, 269, 270 



in nutrition, 258 

 Visceral arches, 187, 188 

 in chick, 218 

 in pig, 10-mm., 272 

 Visceral clefts, 187, 188 

 Visceral groove, 

 in chick, 

 48-hr., 218 

 72-hr., 224 

 in pig, 10-mm., 272 

 Vital staining, in amphibian, 58-61 

 Vitamin E, effect of, 



on chick blastoderm, 264 

 in pregnancy, 264 

 on sperm, 264 

 Vitamin K, 292 

 Vitelline artery, in chick, 173, 174, 178, 



215,216,223, 226 

 Vitelline vein, in chick, 

 48-hr., 220 

 72-hr., 223, 226 



White matter, 162 



Wing bud, of chick, 72-hr., 216 



Wolffian duct (see Mesonephric duct) 



Yolk, 64, 70 



chemical composition of, 203 



in frog, 137 



in growth, 251 

 Yolk sac, 



in chick, 173, 175, 185, 186 



circulation in, 174, 178, 252 



fate of, 185 



function of, 172 



in human, 268, 269, 276 

 Yolk plug, 64 



in frog, 117 



in salamander, 119 



Zona pellucida, 266 

 Zygote (see Fertilized egg) 



