ON THE ANATOMY OF HISTRIOBDELLA HOxMAKI. 343 



would seem to be confined to its anterior region. Fertilisa- 

 tion takes place within the ovarian chamber, as a large 

 oblong sperm-nucleus is always found in the ovarian egg, in 

 which the amphiaster has appeared. This always lies at some 

 distance from the spindle and close to the egg-membrane, 

 while the spindle is centrally placed. I have been unable to 

 determine at just what stage in the growth of the ovariau 

 egg fertilisation takes place. As the egg is seen to increase 

 considerably in size after the amphiaster has appeared, and as 

 the sperm nucleus is always found in the ovum when this is 

 present, it is possible that the egg is fertilised at a stage in 

 Avliich the yolk-granules are first beginning to appear. The 

 fusion of the pro-nuclei takes place only after the polar body 

 is extruded, and this takes place when the egg has been 

 deposited in the sea-water. 



In Stratiodrilus Haswell has observed the fertilisation of 

 the egg taking place within the ovarian cavity. 



In Dinophilus tseniatus, according to Harmer (12), the 

 same conditions hold regarding impregnation and fertilisa- 

 tion as in Histriobdella. The penis is inserted anywhere 

 under the skin, the act of copulation taking place repeatedly 

 with the same female. He says, " the act of copulation has 

 no relation to the maturity of the ova of the female, nor is it 

 prevented by the fact that the female has already received an 

 ample supply of spermatozoa by a preceding operation" 

 (p. 13). The spermatozoa can be seen collected in small 

 masses beneath the gut. Fertilisation is therefore internal. 

 The polar bodies are given off apparently when the eggs reach 

 the exterior, or shortly after they are deposited in the sea- 

 water. 



In the ripe egg, after the amphiaster of the first polar 

 body has been formed in this manner, a remarkable occur- 

 reuce can be brought about, which demonstrates most clearly 

 the semi-solid nature of the spindle itself. In compress- 

 ing the cover-glass on a preparation of a living parasite 

 I happened in several instances to rupture the body-wall in 

 the vicinity of the ovum. The egg-envelope was also broken 



