28 EMBRYOLOGY 



it is cut off into the four lower cells in the vegetal hemisphere. At still 

 later stages — in the blastula, for example- — the pigment band is still in the 

 same relative position, just below the equator of the blastula. The blastula 

 consists of a single layer of cells arranged to form the wall of a sphere, 

 which is filled with a liquid. In Figure 10 a thin slice of the blastula is taken 

 through the pigment band. The diagrams show what happens to the pig- 

 ment band during gastrulation. In the blastula some of the cells at the 

 vegetal pole, V , migrate into the interior. The remaining vegetal cells and 

 the cells containing the pigment band then fold into the interior to form a 

 pocket termed the arcbenteron, or primitive gut. The process of folding, 

 resulting in a double-layered gastrula, is an essential part of gastrulation. In 

 the figure a thin slice of the gastrula shows the external layer of cells, the 

 ectoderm, and the internal layer bounding the archenteron, the endoderm. 

 Thus the cells containing the pigment band fold in to form the walls of the 

 archenteron, the primitive gut, which forms the digestive tract of the pluteus. 

 This primitive gut is made up of endoderm, and therefore the region of the 

 egg containing the pigment band is destined in normal development to 

 become endoderm in the pluteus. 



In the same way we can follow the development of the region at the 

 extreme vegetal pole, V. This region remains in its same relative position 

 at the vegetal pole all through cleavage (2, 4, 8, 16 cells), and even in the 

 blastula stage, it still is an unpigmented region below the pigment band. At 

 the beginning of gastrulation the nonpigmented cells move into the cavity 

 of the blastula and build up a mass of cells on either side of the forming 

 archenteron (Fig. 10, gastrula). These cells give rise to mesoderm, which 

 finally forms the skeleton in the pluteus larva. Thus we can add now that 

 the vegetal region of the egg forms the mesoderm of the larva. Since the 

 mesoderm cells are scattered cells, they are termed mesenchyme. 



The region above the pigment band — a little more than half the egg- — 

 can also be followed through directly to the blastula. It forms a mass of cells 

 in the animal half of the blastula. These cells remain on the outside of the 

 larva and in the gastrula stage they have spread so that they form the entire 

 outer layer of the embryo. Therefore their normal destiny is ectoderm. Thus 

 we have followed one method of determining' the general architecture of the 

 egg by tracing some marker through normal development. And this method 

 demonstrates for us a general organization of the sea urchin egg into regions 

 which give rise to the three germ layers: ectoderm, mesoderm, and endoderm. 



