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| 91. | trophoblast | fluid-filled shell of squamous cells destined to become the chorionic villi, placenta, and associated fetal membranes |
| 92. | true labor | regular contractions that immediately precede childbirth; they do not abate with hydration or rest, and they become more frequent and powerful with time |
| 93. | umbilical cord | connection between the developing conceptus and the placenta; carries deoxygenated blood and wastes from the fetus and returns nutrients and oxygen from the mother |
| 94. | vernix caseosa | waxy, cheese-like substance that protects the delicate fetal skin until birth |
| 95. | X-linked dominant | pattern of dominant inheritance that corresponds to a gene on the X chromosome of the 23rd pair |
| 96. | X-linked recessive | pattern of recessive inheritance that corresponds to a gene on the X chromosome of the 23rd pair |
| 97. | X-linked | pattern of inheritance in which an allele is carried on the X chromosome of the 23rd pair |
| 98. | yolk sac | membrane associated with primitive circulation to the developing embryo; source of the first blood cells and germ cells and contributes to the umbilical cord structure |
| 99. | zona pellucida | thick, gel-like glycoprotein membrane that coats the oocyte and must be penetrated by sperm before fertilization can occur |
| 100. | zygote | fertilized egg; a diploid cell resulting from the fertilization of haploid gametes from the male and female lines |
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