48 



pericardial sac being overspread by them. The whole 

 space between the internal and external faces of the 

 mantle is now filled up by the gonads, and the tubules of 

 the latter are filled, in female and male, with a dense mass 

 of ova and spermatozoa respectively. The spermatozoa 

 are arranged in rows on the radial-supporting filaments 

 already mentioned, and the mutual pressure of the ova 

 against each other in slightly later stages than this gives 

 them a polyhedral form. The germinal epithelium is still 

 undergoing proliferation, but this is not so active now. 

 Where, in the male, the walls of the tubules are closely 

 compressed together, the basement membrane can be seen 

 as a thin, homogeneous line, on either side of which is an 

 irregular row of large, rounded cells, with clear cell bodies 

 and conspicuous nuclei, and internal to this the tubule is 

 filled with a mass of radially-arranged rows of spermatozoa. 

 The conspicuous reticular tissue present in earlier stages 

 is reduced to the merest trace, and is present mostly at 

 the angles formed at the junction of several tubules. It 

 stains less intensely with eosin, but the peculiar vacuo- 

 lated appearance already referred to still exists. Here 

 and there it is present as rounded masses possessing single 

 nuclei, but it is mostly fibrous. 



As in the male, the germinal epithelium still exists in 

 the female, and ova are still being separated off. Most of 

 the eggs lie freely in the cavity of the tubule in contact 

 with each other, but some are still attached to the wall by 

 a short stalk. In some parts the wall of the tubule seems 

 to consist of the basement membrane only. 



Stage IV. (PI. II., fig. 5). — From April onwards there 

 is apparently little change in the appearance of the gonads. 

 The maximum development, as indicated by the increase 

 in mass of the tubules, the numbers of ova and sperma- 

 tozoa liberated from the germinal epithelium, and the 



