1 2 3 4 5

		In the sea urchin, early cell divisions are rapid; the cell cycle alternates between S phase, where new DNA is synthesized, and mitosis. As a consequence, the embryonic DNA is not transcriptionally active; it is not actively "read" to produce new messenger RNAs (mRNAs). The proteins that are synthesized during cleavage utilize mRNAs in the cytoplasm provided by the mother (Fig. 7.30). The first 3 cell divisions bisect the embryo equally (Fig. 8.8, staging series). The first 2 cleavage planes from the "top" (known as the animal pole) to the "bottom" (known as the vegetal pole), while the third runs across the equator and separates the embryo into "animal" and "vegetal" halves. The 4th cleavage is more unusual. The cells in the top half divide equally, but those in the bottom half divide unequally, creating large cells (macromeres) and small cells (micromeres). This is accomplished by one of the centrioles positioning itself in the middle of the cell, so that the spindle is displaced to one side (Fig. 8.10). The cells continue to divide until they form a hollow ball known as the blastula (Fig. 8.11). Each of the cells produces a cilia. At this point, the genome is activated and starts to express new genes. One of these genes codes for a protease that digests a hole in the fertilization envelope; the embryo "hatches" and begins to swim. Shortly after hatching, the descendants of the micromeres at the vegetal end detach from the epithelial sheet and move into the blastocoel (ingression). These are known as primary mesenchyme cells and they form the calcium carbonate spiculesof the larval skeleton (Fig. 8.18, 8.19, 8.20). The decendants of the macromeres thicken to form the vegetal plate, which invaginates to form the archenteronor gut (Fig. 8.17, 8.21). This process is known as gastrulation and, in addition to forming the gut, it results in a multilayered body plan (Fig. 8.16). The archenteron extends by cell rearrangement (Fig. 8.23) and by connections between the cells at the archenteron tip (secondary mesenchyme cells) and the extracellular matrix lining the blastocoel (Fig. 8.25). Once the archenteron reaches the other side, the mouth is formed. Click here to see movie. As the skeleton is laid down, the embryo's shape changes to form the prism and then pluteus larvae(Fig. 8.17).