The portion of the blastocyst called the trophoblast eventually helps form the:

a. head of the baby.

b. legs of the baby.

c. entire baby.

d. nervous system of the baby.

e. placenta.

Birth of identical twins results from unusual events during:

a. egg production.

b. sperm production.

c. early cell divisions after fertilization.

d. ovulation.

e. second trimester.

Arrange the following events in their proper sequence:

a. uterine muscle contractions

b. birth of baby

c. assumption of fetal head-down position

d. expulsion of placenta

e. dilation of cervix

The homeobox-containing genes that are active in development have been highly _______, meaning they have remained relatively unchanged though the course of evolution. These genes contain information for producing _______.

Three tools that work in development across animal species are cell _______, cell _______, and programmed cell ______.

Human development starts at ________ and continues through _______.

A critical early step in development is the establishment of positional information in the embryo. The local concentration of substances called _________ in the embryo helps impart this information.

Morphogens work by bringing about the production of the proteins called _______, which turn _______ on or off.

Development works from the general to the specific. In Drosophila, the morphogen called bicoid operates early in development by specifying the development of _______ and _______ portions of the embryo.

In animal development, the process of cleavage produces a ball of cells with an interior cavity. In the succeeding evelopmental process, called gastrulation, the cells in the embryo migrate to form three layers of cells, the _______, _______, and _______, which in terms of location are the _______, _______, and ________ layers of cells in the organism.