Questions: The pre-embryonic period ends when Multiple Choice the blastocyst implants in the uterus. the zygote undergoes cleavage. a morula is formed. the morula develops a trophoblast.

The answer is the first one: the blastocyst implants in the uterus.

Explanation for each option:

• The blastocyst implants in the uterus. This is the correct answer. The pre-embryonic period, which includes fertilization, cleavage, and the formation of the blastocyst, ends when the blastocyst implants into the uterine wall. This marks the beginning of the embryonic period.

• The zygote undergoes cleavage. This is incorrect. Cleavage is an early stage of the pre-embryonic period where the zygote begins to divide, but it does not mark the end of the pre-embryonic period.

• A morula is formed. This is incorrect. The morula is an early stage in the development of the embryo, occurring after cleavage but before the formation of the blastocyst. It does not mark the end of the pre-embryonic period.

• The morula develops a trophoblast. This is incorrect. The development of a trophoblast occurs as the morula transitions into a blastocyst, but the pre-embryonic period continues until implantation.

In summary, the pre-embryonic period concludes with the implantation of the blastocyst in the uterus.