THE EARTHWORM 



307 



At the time of egg laying, the cHtellum exudes a secretion that 

 becomes a girdle-like structure as it hardens by exposure to the air 



t 



Fig. 148. — Development of earthworm within egg capsule. 



A to C, fertilization ehowing entrance of spermatozoon, approach and fusion of male and 

 female nuclei (rf. Fig. 116, p. 2.3.3 and Fig. 213, p. 40.5). D, two-cell stage. E, early blastula 

 stage showing cells which will give rise to ectoderm, endoderm and mesoderm in later 

 development. F, late blastula showing beginning mesoderm formation. G, gastrula 

 showing three primary germ layers and formation of enteron and coelome. //, embryo 

 showing beginning of adult structures. 



bp, blastopore; br, brain; cw, coelome; ec, ectoderm; en, endoderm; m, mesoderm; 

 n, nephridia; p.h., polar bodies; s, spermatozoon; sc, segmentation or blastula cavity; 

 I'.wi., fertilization membrane; v.n.c, ventral nerve cord. (Based upon figures and descrip- 

 tions by Wilson, Jour, of Morph., vol. 3.) 



(Fig. 147 C). This is gradually slipped forward as a ring might be 

 drawn from a napkin. During its passage toward the head, several 

 pva pass from the opening of the oviduct on segment fourteen into 



