50 W. BALDWIN SPENCER. 



characteristic groups^ in all portions. The walls have the 

 same structure in the main as in the young form, but instead 

 of a complete coating of cells (or syncytium) on the internal 

 surface, there are only present irregularly scattered and 

 numerous multinucleate masses, evidently relics of the 

 original continuous layer. These must be continually budding 

 off in other sperm-cells into the cavity (fig. 21, 7".). 



The testis thus differs in its structure in important points 

 from the ovary of the adult female, though both agree at an 

 early stage of development in having the form of a single 

 median dorsal tube. In the case of the ovary the cells of the 

 walls give rise dorsally to ova which pass outwards, that is 

 away from the central cavity. In the case of the testis, on 

 the contrary, the cells of the walls give rise over the whole 

 surface to sperm mother-cells, and these pass into the cavity, 

 so that the form of a tube with walls of definite outline is 

 retained in the testis and lost in the ovary. 



In the case of the latter, also, at the anterior end its walls 

 and cavity are directly continuous with those of the two ovi- 

 ducts. In the case of the testis the mode of connection with 

 the succeeding portions of the organs — the vesiculse seminales — 

 is of considerable interest when viewed in the light of the 

 probably originally double nature of the testis. It approxi- 

 mates, moreover, in essential features to that already described 

 by Hoyle in P. protelis, and by Stiles in P. probosci- 

 deum. This relationship is shown diagraramatically in fig. 

 68, and Sections A — G. At its very anterior end the wall of 

 the testis is continuous with the posterior wall of a chamber 

 formed by the union in the mid-dorsal line of the two tubular 

 vesiculse seminales. Into this chamber it has a double 

 opening. The anterior wall of the testis is thick and mus- 

 cular, and ventrally there is placed, at the very front end, in 

 the floor of the cavity, a well-marked double muscular ridge. 

 This is well seen in transverse sections (fig. 18, V. S. O.). 

 Each half of the ridge ends in a papilla, on which is a funnel- 

 shaped opening, the broad end leading into the testis cavity, 

 the narrow one into a small canal which leads through the ridge 



