Final answer:
The formation of the brain from the neural tube begins during the third week after fertilization when the neural plate folds to form the neural tube. By the 25th day, the anterior end of the neural tube starts developing into the brain, leading to the establishment of complex brain structures in the coming weeks.
Step-by-step explanation:
The process of the brain's formation begins with neurulation, which occurs in the third week after fertilization. During this critical phase of development, the neural tube is formed from a structure called the neural plate, which originates from the ectoderm layer of the embryo. This transformation includes the neural plate folding inward and its borders converging to establish the neural tube beneath the ectoderm. By the 25th day post-fertilization, the anterior portion of the neural tube starts developing into the brain. This process sets the foundation for the complex structures of the nervous system and continues into the fourth week of development when the anterior neural tube further subdivides and dilates to form vesicles that differentiate into distinct regions of the brain.