Fig. 2, Complete transverse section through the body of 

 an adult female Mussel passing through the peri- 

 cardial cavity just posterior to the heart. Note the 

 presence of ovarian tubules in every part of the 

 section, the stage of development being the same 

 both in mantle lobes and in visceral mass. The 

 specimen was taken about the end of September, 

 and proliferation of ova from the germinal 

 epithelium is beginning, but the diameter of the 

 tubules is still very small. X 5 diameters. 



Plate II. 



Fig. 1. Transverse section of mantle of nearly ripe female 

 Mussel from Koosebeck outer scar, taken during 

 March. The mantle is nearly filled with the 

 tubules of the gonad containing many free ova. 

 The germinal epithelium is still undergoing 

 proliferation. X 40 diameters. 



Fig. 2. Transverse section of mantle of nearly ripe male 

 Mussel from Roosebeck outer scar. (March). 

 The mantle is filled with sperm tubules. X 40 

 diam. 



Fig. 3. Transverse section of mantle of female spent 

 Mussel from New Brighton beds, taken during 

 August. The tubules of the gonads have in many 

 cases broken down. Two are shown in the section, 

 one containing an egg which has not been 

 extruded during spawning. Note the great 

 development of the reticulum. X 200 diam. 



Fig. 4. Transverse section of mantle of spent female 

 Mussel from the beds in the Barrow Channel 

 (August). A blood space lying immediately 

 beneath the epidermis is shown, beneath which 

 are two collapsed ovarian tubules. Note that 



