240 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
the duct, as in the alveolus, this only forms a small strip 
of the wall, the rest being composed of germinal epithelium. 
In the female gonad (fig. 38), at a corresponding stage, 
the cavities of the ducts and alveoli are filled with eggs in 
various stages of development. As in the male gland there 
is a strip of the wall which in the alveolus is thin and 
membranous, and in the ducts is composed of a ciliated 
epithelium; the remainder of the duct consists of germinal 
epithelium which, in some parts, is composed of small, 
deeply staining cells with conspicuous nuclei. Many of 
these are enlarged and project out from the wall into the 
lumen; the largest eggs which are attached have a narrow, 
short stalk which forms part of the wall. The eggs lying 
freely in the cavity are flattened against each other by 
their mutual pressure, and are usually polygonal in section. 
Their true shape is oval (fig. 35, Pl. VI.). A very thick 
vitelline membrane (M.vzt.), secreted apparently from the 
surface during the later stages of development, surrounds 
each. The cell contents are coarsely granular. The nucleus 
is a large, oval body, with a very sharp outline, faintly 
granular in texture, and with no apparent signs of chromatic 
material. A single, large, rounded, very distinct nucleolus 
is always present, the contents of which sometimes show 
a very regular vacuolation; within the ovary the eggs are 
frequently adherent together by their membranes. 
The eggs and spermatozoa are shed in the early part of 
the year (March), and spawning apparently lasts till July 
or August. Fertilisation and development take place at 
large in the surrounding water, resulting in the formation 
of a typical veliger larva. After a short free-swimming 
stage, the velum is absorbed, the shell is formed, and the 
long vermiform foot is developed. The young cockle, 
then still less than 1 mm. long, settles down in the sand 
for the remainder of its existence, 
