COE : NEMERTEANS OF WEST AND NOKTHWEST COASTS. 65. 



This duct does not pierce the outer layers of the body wall until the 

 sexual products are fully mature. These products are discharged 

 by being merely pressed through the efferent ducts, when they fall 

 into the water outside the body. Their discharge is under the 

 control of the animal, however, and under circumstances they may 

 be held in the body for a long time unless the worm receives the 

 necessary stimulus. 



A few species, belonging to mdely separated genera, are vivip- 

 arous, the eggs being fertilized within the body. 



In Cerebratulus, Drepanophorus, certain species of Amphi- 

 PORUS and probably numerous other forms belonging to the higher 

 orders, a small portion of each sexual gland remains after the 

 genital products have been dischai'ged and can be distinguished 

 at all seasons of the year. In these forms the process of develop- 

 ment of the sexual products may be followed with comparative ease. 



Ovogenesis — As described in the account of Amphiporus gelati- 

 nosus, the thin-walled follicle remains as a small sac lined with flat- 

 tened cells immediately after the last sexual elements have been dis- 

 charged. Tlie gland remains in this condition until the season for 

 the produ^ction of the sexu.al elements. In the case of the female 

 certain of the nuclei then increase rapidly in size and may be dis- 

 tinguished as the germinal vesicles of future ova, while neighboring 

 nuclei become surrounded with yolk spheres (PI. 20, figs. 125-127). 

 The details of the formation of the yolk-niicleus and the develop- 

 ment of the ovum are likewise described for A. gelatmosus. 



A similar process has been described for Cerebratulus by C. B. 

 Wilson (: 00, p. 123), and hei'e the position of the stalk of attachment 

 can be distinguished even after the egg leaves the body. The ger- 

 minal vesicle always occupies the side of the egg opposite the stalk, 

 the polar bodies form in the vicinity of the germinal vesicle, and the 

 plane of the first cleavage extends vertically through both the region 

 of the polar bodies and that of the egg stalk. The egg can thus be 

 definitely oriented even before it leaves the ovary. 



Mature eggs are either spherical or somewhat oval, and vary 

 greatly in size, color, opacity and general appearance in different 

 species. They are always provided with a large germinal vesicle 

 which varies from one third to one half the diameter of the egg. 



In Carinella frenata 20-50 ova develop in each of the closely 

 packed genital sacs. These eggs are opaque and rose colored, giv-^ 



