STEUCTUEE OF EARTHWOEM ALLIED TO NEMEETODRILUS. 569 



In transverse sections the egg-sac of each side is seen to be a 

 spherical body lying within the spermathecal sac, and supported 

 at one side by the wall of this sac ; its cavity, however, is 

 nowhere continuous with the cavity of the spermathecal sac ; 

 the two structures are so far perfectly independent, and appear 

 to be as it were accidentally associated. I shall show later 

 (on p. 574) that there are also embryological reasons for 

 regarding the egg-sac as entirely unconnected with the great 

 unpaired sac in which it lies. The egg- sac of all earthworms 

 in which it has. been found, with the exception of the Eudrilidse, 

 lies in the 14th segment, attached to the anterior wall of this 

 segment. In Libyodrilus there is a septum separating the 

 I3th from the 14th segment; but this septum does not traverse 

 the spermathecal sac ; hence the egg-sacs do not depend from 

 any intersegmental septum in the fully mature worm. 



In its microscopical structure the egg-sac presents no pecu- 

 liarities of any particular interest. Its cavity is, as is usual 

 with this organ, subdivided into numerous chambers, packed 

 with ova and germinal cells in various stages of development. 

 The walls of the sac are formed largely of muscular tissue with 

 abundant nuclei ; the compartments which are formed by in- 

 growth of this layer are also muscular, but seem to be irregularly 

 lined with granular peritoneal cells. The relations of the egg- 

 sac to the spermathecal sac are shown in fig. 12; it will be 

 noticed that it projects into the interior of that sac, and in other 

 sections, taken at a different level from the one figured, it 

 appears to lie freely in the interior of the sac. It is, however, 

 easy to see from the histology of the parts concerned that the 

 egg-sac is not really within the spermathecal sac ; the walls of 

 the latter are not perforated ; they are merely pushed in by 

 the egg-sac : these points will be understood by a reference to 

 fig. 12. 



The figure shows the egg-sac just where the oviduct opens 

 into it ; two parts of the oviduct are shown ; the lettering od 

 is placed in the oviduct where it commences to widen out into 

 the funnel; to the right of the figure lies a section of the 

 narrower part of the tube. Running round the walls of the egg- 



