COE: NEMERTEANS OF WEST AND NOKTHWEST COASTS. 



73 



egg, and formation of the pilidium of Cerebratulus inarginatus, 

 which occurs on the Pacific coast, may be given here. ■ The mature 

 egg of this species measures about 0.16 mm. in diameter, with an 

 eccentrically placed germinal vesicle of about one third to two 

 fifths the diameter of the egg. Soon after the egg is discharged 

 into the;water its germinal vesicle disappears and the polar spindle 

 is formed with 16 small chromosomes in its equatorial plate. The 

 developmental processes will go no further, however, until after the 



M' 



Fig. 29. — Cerebratulus marffinotus. Median sagittal section of embryo at begin- 

 ning of pilidium stage ; .s/j, apical plate ; fi, llagelhim ; sc, segmentation cavitj- ; 

 St. stomach, showing four deeply stained gland cells (ffl); e, esophagus; m, 

 mouth; II, lappet ; ec, ectoderm ; 7ns, mesoderm. 



entrance of the spermatozoon, and in eggs which are not fertilized 

 the first polar body is not formed. The spermatozoon may enter 

 the egg at any point, but usually does so at the side of the egg 

 farthest from the eccentrically placed germinal vesicle. The pair of 

 centrosomes and asters fonm in the immediate vicinity of the spei'm 



