50 



that it has spread into the spaces surrounding the tubules 

 of the gonad from other parts of the body. Round each 

 tubule this reticulum forms a dense mass, with few or no 

 interspaces, and its syncytial character is here strongly 

 suggested. The nuclei are not so evident as at a later 

 stage. Within the tubule itself the linear arrangement of 

 the spermatozoa is very striking, but instead of being 

 directed towards the centre these rows point towards that 

 part of the wall lined with the strip of ciliated epithelium 

 already described (PL II., fig. 5, ep. cil.). Often a blood 

 space (bl. sp) is situated beneath this epithelium; over 

 the cilia and within the tubule is a space free from sper- 

 matozoa. The wall of the tubule has become indistinct 

 from the mass of spermatozoa which seem to abut directly 

 on the tissue of the reticulum {ret.) surrounding the tubule. 

 The whole arrangement strikingly suggests the exercise of 

 pressure on the tubule by the rapidly developing reticulum 

 and the removal of the genital products brought into close 

 contact with the ciliated strips of the tubules. The lower 

 conducting portions of the gonads are entirely lined with 

 ciliated epithelium, and once here the genital products 

 must be speedily removed. 



Briefly summarised then, the history of the yearly change 

 in the reproductive organs of the Mussel is as follows : — 

 (1) There is a short period of rest following spawning, and 

 occupying some part of August and September, during 

 which the space in the body of the animal formerly 

 occupied by the gonads is largely filled up by a large-celled 

 reticulum. This is followed by (2) the reformation of the 

 germinal epithelium and a slow proliferation of ova and 

 spermatozoa, lasting for the rest of the year. During this 

 period the gonads increase greatly in mass, and invade the 

 mantle and other parts of the body, displacing the reticu- 

 lum formed during spawning. Succeeding this is a period 



