THE ANATOMY OP SCALIBREGMA INFLATUM. 285 



septum. These cells are the very young reproductive cells. 

 They are found only in the region of the smaller lip of the 

 nephrostome, and for a little distance along the outer side of 

 the straight tube of the nephridium where this organ is in 

 contact with the rudimentary septum. Gonads are present 

 on all the fully grown nephridia^ but those on the first and 

 second nephridia are very small. The gonads connected 

 with the first three nephridia ai-e situated on the smaller lips 

 of the funnels and on the neighbouring portions of the an- 

 terior faces of the last three diaphragms. 



The nuclei of the reproductive cells are large and deeply 

 staining. 



It is not possible^ I think, to distinguish the sex of a 

 specimen by examination of the cells of the gonad, because 

 they are shed into the ccelom when so small that they have 

 not become sufiiciently differentiated to be recognisable as 

 either young ova or young spermagonia. It is only after an 

 examination of the more mature sexual products usually 

 found in the coelom that the sex of the specimen can be 

 determined. 



The reproductive cells leave the gonad when about 10 — 

 12 /i in diameter. They increase in size in the coslom, and 

 by the time they have reached 15^ in diameter their nature 

 may be determined, as in males division of the cells now 

 takes place, and in females the cells are recognisable as 

 young ova. I'he ovum continues to grow in size, and its 

 protoplasm — which up to this point has been clear and homo- 

 geneous — gradually becomes loaded with spherical yolk 

 granules about 1 /.t in diameter, Avhich stain deeply with 

 hasmatoxylin. The largest ova in my specimens are '12 mm. 

 in diameter; these are probably almost ripe. The nucleus is 

 excentric and vesicular, about 30 ju in diameter, and has a 

 prominent deeply staining nucleolus. The peripheral layer 

 of protoplasm is almost free from yolk granules, but these 

 granules are moderately uniformly distributed throughout 

 the other parts of the protoplasm. The vitelline membrane 

 of these ova is thin. 



