EMBRYOLOGY OF THE EARTHWORM 



155 



openings of the sperm receptacles. When the girdle passes off 

 the anterior end it closes at the front end and afterward at the 

 posterior end. The girdle thus forms a coccoon which hardens 

 later into a chitinous spindle-shaped vessel containing re- 

 productive cells and albuminoid food material (Fig. 65). The 

 eggs are fertilized in the coccoon by the spermatozoa and de- 

 velopment begins at once and continues under the protection of 

 the coccoon. 



Cleavage of the egg is irregular to the i6-cell stage with four 

 vegetative cells at the lower, and smaller animal cells at the 

 upper pole. The lower cells invaginate and form a typical two- 

 layered gastrula. Up to this stage development closely follows 

 the type described on page 78. but from here on it becomes 



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

 by on the right enlarged to the same scale); J5, an ovum highly magnified; C, a 

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

 and /, tail. (From Sedgwick and Wilson.) 



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 

 cavity. In the meantime the embryo has elongated in the 

 main or antero-posterior axis passing through the blastopore, 

 and new ectoderm cells are pushed in from the ectoderm andr 

 secondary mesodermal pole cells are separated from the meso- 

 derm. The former are the first stages of the nervous system 

 and are known as neuroblasts. The latter are of different kinds 

 with different functions to play later. Some are muscle-form- 

 ing cells called somatoblasts , and some are nephridia forming 



