EMBRYOLOGY OF THE EARTHWORM 



155 



openings of the sperm receptacles. When the girdle passes off 

 -:he 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 

 1 he coccoon. 



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

 ^egetative 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 

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

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

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



complicated by the formation of a third germ layer called the 

 riesoderm. This arises from two pole cells coming from the 

 \ egetative 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 

 riain or antero-posterior axis passing through the blastopore, 

 and new ectoderm cells are pushed in from the ectoderm and 

 s xondary mesodermal pole cells are separated from the meso- 

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

 a nd are known as neuroblasts. The latter are of different kinds 

 v ith different functions to play later. Some are muscle-form- 

 ing cells called somatoblasts , and some are nephridia forming 



