EMBRYOLOGY OF THE EARTHWORM 



159 



layered gastrula. Up to this stage, development closely follows 

 the type described on page 79, but from here on, it becomes 

 complicated by the formation of a third germ layer, called the 

 mesoderm. This arises from two pole cells, coming from the 

 vegetative pole and taking an initial position in the segmenta- 

 tion cavity (Fig. 66). They then divide, forming a sheath of 

 cells on each side of the median line and filling the segmentation 



FIG. 65. A, Egg capsule, enlarged five diameters (a few eggs, ov., are shown 

 near by on the right enlarged to the same scale); B, an ovum highly magnified; 

 C, a spermatozoon still more highly magnified; w, nucleus or head; m, middle 

 piece; and t, tail. (From Sedgwick and Wilson.) 



cavity. In the meantime, the embryo has elongated in the 

 main or antero-posterior axis passing through the blastopore; 

 new ectoderm cells are pushed in from the ectoderm, and sec- 

 ondary mesodermal pole cells are separated from the mesoderm. 

 The former are the first stages of the nervous sytem, and are 

 known as neuroblasts. The latter are of different kinds, with 

 different functions to play later. Some are muscle-forming 

 cells called somatoUask, and some are nephridia-forming, 

 known as nephroblasts. All give rise to sheets of cells which be- 

 come differentiated into the ultimate adult structures nervous 

 system, muscles and nephridia. 



Meanwhile, the masses of mesoderm cells on each side of the 

 median line begin to show traces of a loose structure, and later, 

 well-marked spaces or cavities are developed from these spaces 

 and assume regular shapes. They are first clearly formed in 

 the region of the blastopore, as regularly arranged cavities lined 

 by mesoderm cells. These cavities are the coelomic cavities of 

 the adult, and their anterior and posterior walls form the dis- 



