EMBRYOLOGY OF ECHDfODERMS. 



107 



hours, according to the temperature, the eggs present the 

 appearance shown in Fig. 56. Careful examination, in a 

 good Ught, will now show that the egg is hollow, and 

 consists of a spherical shell (Fig. 57, h), around a central 

 space, or segmentation cavity, i. The shell consists of a 

 single layer of wedge-shaped spherules or cells, each of 

 which contains a nucleus. 



Fig. 57. 



Fig. 58. 



Fig. 57. — Diagram of the same egg, seen in section. 



h. The single spherical layer of cells, i. The segmentation cavity. 



Fig. 58. — Diagram of the embryo as seen in section at the beginning 

 of the gastrnla stage. 



i. Segmentation cavity, j. Ectoderm, k. Endoderm. m. Orifice of 

 invagination. 



m. The Gastrula stage. One side of this shell now 

 becomes invaginated, or pushed in towards the other, as 

 shown in Fig. 58, thus forming a second cavity, m, the 

 primative digestive ca\-ity, which opens externally by a 

 large, funnel-shaped orifice, the gastrnla mouth, or orifice 

 of invagination. As the direction cell does not persist in 

 Arbacia, the relation between the principal axis and the 

 ingrowth cannot here be made out, but the analogy of 

 other animals gives great reason to believe that the invagi- 

 nation takes place along the principal axis, but at the 

 nutritive pole or opposite the direction cell. The layer of 

 cells is now divisible into two portions : the endoderm (^) , 



