COE: TERRESTRIAL NEMERTEAN OF BERMUDA. 549 
This ovum is surrounded by the flattened follicular epithelium of 
the gonad to which it remains attached for some time at its broader 
end. It then breaks off and lies free in the ovary, which has pre¬ 
viously formed its duct to the exterior. The entrance of a sperm 
cell from another individual into the ovary fertilizes this egg cell, 
which then develops into an embryo, while the duct to the exterior 
of the body closes up, as described above. 
The mature eggs measure about 0.35 to 0.45 mm. in diameter. 
Compared with the diameter of the body of the worm they are 
extremely large, being in many instances from one third to one half 
as great as the width of the body. Consequently they are arranged 
in a single row on each side. In the stained sections of the body 
they are conspicuous even to the unaided eye. They have a pale 
yellow color in life. Each egg is surrounded with a delicate mem¬ 
brane of follicular cells and crowded with spherical yolk granules of 
various sizes, some of which are as much as 0.013 mm. in diameter. 
In G. australiensis the eggs are nearly one third as large as the 
diameter of the body of the worm, and do not all mature at the same 
time. They are fertilized within the body as in G. agricola , but 
are deposited before cleavage. The spermatozoa may remain alive 
for some time after their entrance into the ovary and may fertilize 
an ovum which matures some days, or perhaps weeks, later. In this 
way clusters of fertilized eggs may be deposited at intervals of a 
week or more. It seems probable that in G. agricola a portion of 
•the eggs may likewise mature before the others, for both eggs and 
embryos in the body exhibit considerable variation in size. 
Spermatogenesis .— In the sexual glands which are to form both 
ova and spermatozoa the latter develop out of the undifferentiated 
germinal cells which make up the substance of the ovo-testis. As 
stated above, the spermatozoa usually mature and are discharged 
before the maturity of the ova in the same gonad. The course of 
development of the spermatozoa is apparently the same whether they 
are formed in an ovo-testis or in special gonads which develop 
spermatozoa only. 
In the young spermary comparatively large cells only are present. 
These divide many times to form the last generation of sperma¬ 
togonia, which by two divisions give rise to the spermatids. 
In spermaries which are approaching maturity, as shown in figure 
26 (pi. 25), it is easy to distinguish three kinds of cells in addition 
