Gross Development of the Human Nervous System
When an egg is fertilized by a sperm, the resulting human zygote consists of just a single cell. But this cell soon begins to divide, and by the 15th day, the emerging
embryo resembles a fried egg, as shown in Figure 6-5. It is made of several sheets of cells with a raised area in the middle called the embryonic disc, which is essentially the primitive body.
By day 21,3 weeks after conception, primitive neural tissue, known as the neural plate, occupies part of the outermost layer of embryonic cells. The neural plate first folds to form the neural groove, as detailed in Figure 6-6. The neural groove then curls to form the neural tube, much as a flat sheet of paper can be curled to make a cylinder.
- Figure 6-3
Embryos and Evolution
The physical similarity of embryos of different species is striking in the earliest stages of development, as these salamander, chick, and human embryos show. This similarity led to the conclusion that embryos are not simply miniature versions of adults.
Prenatal Stages
Zygote = fertilization to 2 weeks Embryo = 2 to 8 weeks Fetus = 9 weeks to birth
Head
Brainstem
Neural tube (forms spinal cord)
Hindbrain
Brainstem
Forebrain
Neural tube (forms spinal cord)
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