In an embryo, nerve cells grow out from the spinal cord and form connections with the muscles they will eventually control. What mechanisms described in this chapter might explain how these cells 'know' where to go and which cells to connect with?

In humans, identical twins are possible because

a. Extraembryonic cells interact with the zygote nucleus.

b. Convergent extension occurs.

c. Early blastomeres can form a complete embryo if isolated.

d. The gray crescent divides the dorsal-ventral axis into new cells.

Cells transplanted from the neural tube of a frog embryo to the ventral part of another embryo develop into nervous system tissues. This result indicates that the transplanted cells were

a. Totipotent.

b. Determined.

c. Differentiated.

d. Mesenchymal.

You obtain an egg cell from the ovary of a white mouse and remove the nucleus from it. You then obtain a nucleus from a liver cell from an adult black mouse. You use the methods of nuclear transplantation to insert the nucleus into the empty egg. After some prompting, the new zygote divides into an early embryo, which you then implant into the uterus of a brown mouse. A few weeks later, a baby mouse is born. What color will it be? Why?

In an experiment, a researcher colored a bit of tissue on the ­outside of a frog gastrula with an orange fluorescent dye. The embryo developed normally. When the tadpole was placed under an ultraviolet light, which of the following glowed bright orange? (Explain your answer.)

a. The heart

b. The pancreas

c. The brain

d. The stomach

Should parents undergoing in vitro fertilization have the right to choose which embryos to implant based on genetic criteria, such as the presence or absence of disease-causing genes? Should they be able to choose based on the sex of the embryo? How could you distinguish acceptable from unacceptable criteria? Do you think such options should be legislated?