SEX DETERMINATION IN DINOPHILUS GY ROCILTATUS. 363 
when they are discharged by the female. In this case the 
sperm nucleus does not attempt to fuse with the female 
nucleus until this has undergone reduction, which only takes 
place when the eggs are laid and have come in contact with 
the sea-water. I was not able to determine the exact stage at 
which the sperm enter these eggs in Histriobdella, but it is 
obvious that the condition in Dinophilus is simply a more 
extreme example of the same thing, where the sperm join 
the female germ-cells as soon as these arise in the ovary. 
The maturation divisions, again, in the case of the female 
egg of Dinophilus, only take place after, not before, ferti- 
lisation, which seems contrary to all known rule. In the case 
of many partlienogenetic eggs, however, we know that, 
although maturation divisions take place, these are not 
accompanied by an actual reduction in the number of 
chromosomes, so that their presence in these eggs would 
seem as unnecessary as in Dinophilus, where, if there is 
any reduction, it takes place after fertilisation. In the face, 
however, of these startling facts of early fertilisation and 
maturation after fertilisation in the case of the female egg, I 
have long delayed the publication of the present work, and 
have repeated all my observations again and again in the 
hope of arriving at conclusions more in keeping with orthodox 
tradition. 
I know that it will be pointed out that the small accessory 
chromatic body of the oogonial cells, which I identify as the 
sperm head, is remarkably like the peculiar accessory nucleus 
that is so frequently present during the oogonial and oocyte 
stage of many eggs, and which have been so well figured in 
the papers of Obst (12) and Bambeke (1). The fact, however, 
that in Dinophilus these bodies never attain the size or 
shape of true accessory nuclei, and are within the nucleus and 
never in the yolk or cytoplasm, seems to prove that they are 
not of this nature or related in any way to the large crescentric 
yolk nuclei on many eggs. Their staining reaction, again, is 
always similar to that of the female chromatic substance, 
while their absence from the oogonial cell when early ferti- 
VOfi. 57, I’ART 3. — NEW SERIES. 26 
