SEX DETERMINATION IN DINOPH1LUS GYROCILIATUS. 351 
of these granules seem somewhat refractive and larger than 
the others. They appear to be the forerunners of the germ- 
cells, but at this stage they can hardly be said to differ from 
the ordinary cytoplasmic granules. In the middle of the 
ovary, collected together in a small mass, are the sperms, 
which in the living condition are actively motile. At a stage 
somewhat later than that shown in fig. 25, a few of the 
granules already mentioned are seen to have enlarged, and 
are the first definite oogonial cells. As far as I have been 
able to determine, these arise anywhere in the ovarian tissue 
and not in a more or less restricted portion of it, as in D. 
vorticoides and D. toeniatus. Once the oogonial cells 
appear, they increase very rapidly in numbers and assume a 
characteristic round shape. They have large darkly staining 
nuclei, surrounded in the early stage with hardly any cyto- 
plasm. They soon fill up the whole ovary, but are most 
numerous in the anterior region, where they seem to be 
undergoing very rapid proliferation. At a slightly later date 
it can be seen that they are dividing solely in this anterior 
region, while towards the middle and posterior region they 
are not dividing, but are increasing in size, so there is a 
graduation in size of the oogonial cells from the minute 
almost granular condition at the anterior end, to the large 
cells that are almost oocytes in the posterior region. This 
stage is shown in Text-fig. 2, which is drawn from a com- 
pressed aceto-carinine preparation ; therefore the cells are 
forced out of position and their relative position distorted. 
With the appearance of the oogonial cells and their rapid 
multiplication, the sperms, which have formed a compact 
mass up till this time, become dispersed throughout the 
organ, a large number being pushed up in the augle between 
stomach and gut (fig. 9). If an aceto-carmine preparation is 
made of a female at this stage, one sees a picture something 
like that shown in fig. 20. This represents a preparation of 
the ovary compressed under a cover-glass, as seen under an 
oil-immersion lens. Inside the limiting membrane the organ 
is seen crowded with sperm. The female germ-cells are still 
