ON DIPLOZOON NIPPONICUM X. SP. 187 



well. In this granular protoplasm there are fibrons structures radiating 

 from the central nucleus to the cell- wall and more or less forminsr a 

 net-work. The steps hy which these cells are changed into ripe yolk- 

 cells and the origin of the deeply stained young yolk-cells I must 

 leave at present unexplained. 



As will he immediately seen from the above investigation, the 

 union of Diplozoon is, as Zeller maintains, a permanent copulation. 

 But the relation in which he has represented the parts of the two 

 individuals to stand to each other reqaii-es correction. We have seen 

 that the vas deferens of one individual opens into the yolk-duct of 

 the other. This is well in accordance with the probable mode of 

 copulation in some allied forms. In ]\[icrocotiile, which seems to be 

 very closely allied to Diplozoon, there is a dorsal vagina which leads 

 into a canal opening into the yolk-duct. In this canal I have often 

 observed spermatozoa, and as during the period of reproductive activi- 

 ty yolk-cells are constantly going down the yolk-duct and push down 

 before them anything that might come u}) from below, it is very prob- 

 able that these spermatozoa had found their way here from the dorsal 

 vagina. Hence the supposition is very natural that in copulation the 

 penis of one worm is directly applied to the dorsal vaginal opening of 

 the other. Now if this very probable supposition be true, and if 

 we further imagine such a relation to persist permanently, we 

 should have j List the case that we actually see in Diplozoon, with the only 

 difference that the copulation is not cross-wise. Whether in Micro- 

 cotijle also, as in Poliistoniiun, the copulation is normally cross-wnse 

 and mutual is well worthy of our attentive observation, since if this 

 be the case, the copuhition of Diplozoon would l)e nothing more or 

 less than the regular mode of copulation in. allied forms made per- 

 manent. 



