158 EDWIN 8S. GOODRICH 
distinct layers : an outer thin refringent membrane, the original 
vitelline membrane ; and an inner much thicker and probably 
less dense perivitelline layer (fig. 10). A full-grown ovum with 
its covering is about 120 to 130 microns in longest diameter. 
Young Trophozoites.—The youngest stages of the 
parasite observed were small rounded trophozoites embedded 
in the egg close to its nucleus. Tig. 1 shows such a stage 
where the gregarine is 12 in diameter ; far smaller than the 
nucleus of the immature egg it has invaded, and indeed only 
about twice the diameter of its nucleolus. It will be noticed 
that even at this early stage the nucleus of the parasite is 
distinguished from that of the ovum by the possession of two 
karyosomes, while the latter is almost invariably provided with 
only one nucleolus. The trophozoite continues to grow at the 
expense of the egg, enlarging and becoming stored with granules 
of paraglycogen (figs. 2, 3). As it acquires the shape and size of 
the adult (fig. 7) the egg and its nucleus become more and more 
compressed against the surrounding membranes. 
Penetration into the egg.—lt has been stated above 
that the ovum of Arenicola is protected not only by a vitelline 
membrane, but also when full-grown by a thick perivitelline 
layer. How does the parasite penetrate into the egg? is a 
question which at once suggests itself. Now it is probable that 
fully-developed eggs are safe from imvasion, since infected eggs 
are rarely, if ever, found with the perivitelline layer fully formed. 
By far the greater number of eggs infected are provided with 
a vitelline membrane only (figs. 4, 7), or with but a thin peri- 
vitelline layer as well (figs. 6, 11). The parasite enters the egg 
by boring a round hole through these membranes, and usually 
the margin of the hole is found turned inwards (figs. 2, 4). The 
aperture so formed may remain open ; but sometimes it seems 
to close up almost entirely (fig. 6), presumably when the egg is 
invaded at a very young stage. 
Position and growth of trophozoite in eggs. 
—lIt is often very difficult to decide whether the parasite, having 
pierced the egg-membranes, really enters the egg-cell or merely 
bulges into it. Except perhaps in the very earliest stages it 
