HABITS, ETC., OF OEREBRATULUS LAOTEUS. 125 



The ripe ovum is spherical, quite large (0"1 to 0*2 mm. in 

 diameter), and appears white to the naked eye, but by trans- 

 mitted light is seen to be dark brown and opaque, owing to 

 the large number of yolk granules which it contains. It 

 possesses a germinal vesicle fully one third its own diameter, 

 much lighter in colour, translucent, and slightly elliptical in 

 outline. 



Inside the germinal vesicle is a large spherical nucleolus, 

 which is nearly always eccentrically placed at one end of the 

 vesicle, the end farthest from the original point of attach- 

 ment (ec/", fig. 65). 



As the eggs are developed the ovaries increase greatly in 

 size, until finally they occupy nearly all the space, and the 

 caeca are flattened between them. 



Formation of Oviduct. — The ovaries now push inward 

 nearly to the wall of the intestine, and outward, downward, 

 and upward to the body-wall. In both horizontal and verti- 

 cal sections they appear larger at either extremity than in 

 the middle. 



When the eggs are nearly mature each sac pushes out into 

 the longitudinal muscle layer, presumably at the point where 

 it meets least resistance, i. e. on the dorsal surface about one 

 quarter of the diameter of the body from its lateral edge. 



It then penetrates the muscles as a fine canal, the first 

 beginning of a genital duct, which as soon as the eggs or 

 sperm are fully ripe pushes through the skin to the exterior. 

 The epithelium lining this canal is made up of flattened cells, 

 which are elongated until they look almost like muscle- 

 fibres. 



Just inside the mouth of the duct there is a cluster of 

 gland cells, which secrete a large amount of mucus. 



As maturity advances the ducts grow shorter, in conse- 

 quence of the distension of the body and the resultant de- 

 crease in thickness of the body-walls. Hence when the 

 genital products are ripe they are easily discharged through 

 these ducts. 



As soon as the egg enters the water it is seen to be sur^ 



