63 



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, PI. VI.). A very thick 

 vitelline membrane (M.vit.), 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. 



