2. fetus B. extends from the start of the fourth month to the end of the sixth month.
3. fetal period C. the blastocyst adheres to the uterine lining and sends out projections that invade the mother's tissues.
4. first trimester D. the time span from the third to the end of the eighth week of pregnancy.
5. second trimester E. defined by a trophoplast, a blastocoel, and an inner cell mass
6. third trimester F. the first three months of pregnancy.
7. human blastocyst G. term for the new, distinctly human individual at the end of the embryonic period.
8. implantation H. from the start of the ninth week until birth, organs enlarge and become specialized.
Arrange the following human developmental stages of early human embryo formation in the proper chronological sequence (day 1 to day 14). The letter of the first stage will be number 1 and the letter of the final process will be number 9.
1.___ A. a blastocyst develops.
2. ___ B. the four-celled stage is completed by the second cleavage division to produce four
3. ___ C. cleavage begins about 24 hours after fertilization.
4. ___ D. a connecting stalk has formed between the embryonic disk and chorion; chorionic
villi begin to form.
5. ___ E. blood-filled spaces begin to form in maternal tissue; the chorionic cavity starts to
6. ___ F. cleavage divisions have produced a ball of six to twelve cells.
7. ___ G. the yolk sac, embryonic disk, and amniotic cavity have started to form.
8. ____ H. a morula, a ball of sixteen to thirty-two cells, is produced; the morula gives rise to
the embryo and attached membranes.
9. ____ I. the blastocyst begins to burrow into the endometrium.
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