464 V. A. I'OTTS. 



goniidial tube is so small in comparison with the size of the 

 egg that the growing oiicytes are arranged in a linear series. 

 The oocyte nearest maturity is just posterior to the sperm- 

 forming region, and behind it is a line of developing 

 egg-cells showing the stages ot" growth from tiie scarcely 

 differentiated oogonin. The spermatozoa as fully formed are 

 small circular discs, capable of amceboid movements when 

 effecting fertilisation. They remain in the i-egion of the 

 gland where they were formed, so that what was testis in 

 the first stiige becomes receptaculuni scruinis in the 

 second. In its formation, since the spermatozoa occupy a 

 much smaller bulk than the spermatocytes, the recepta- 

 culuni seminis shortens considerably ; its epithelium is of 

 course the investing layer of the testis. The spermatozoa 

 are now so disposed that the ripe ovum can pass out of the 

 ovary and through the receptaculuni seminis without its 

 motion being impeded. During its passage a single sperma- 

 tozoan fuses with the esrsr-cell and brink's about fertilisation. 

 The fertilised egg immediately becomes enveloped by a 

 cuticular shell, and lies for some time in the uterus under- 

 going segmentation before it is finally ejected to the exterior 

 by the pressure of eggs from behind (Text-fig. 7, c). The 

 formation of ripe eggs after the first is perfectly regular, and 

 fertilisatio:i occurs in every case. Since, then, the whole 

 quantity of spermatozoa is formed before the first egg is 

 i-eady for fertilisation, it follows that a limit is set to the 

 nunibei' of fertile eggs it is possible to produce, and as a 

 matter of fact this limit is reached at a compai-atively early 

 point in maturity. When the receptaculuni seminis is 

 completely emptied of its spermatozoa eggs still continue to 

 be laid at a uniform i-ate, though they never develop to larva;. 

 Ill Diplogaster maupasi (Text-fig. 8) events follow a 

 very similar course. There is, indeed, one difference in detail 

 during the early })eriods of egg-laying which may be briefly 

 mentioned. The proximal limb of the gonad is shorter, the 

 distal longer than usual. The former is entirely occupied by 

 the uterus and testicular region, and the ovaiy is confined to 



