of the Fishery Board for Scotland. 229 



Fertilization. — M'Intosh and Wilson believe that the fertilization of 

 the eggs takes place outside the body of the female. 



Wilson effected artificial fertilization of the eggs by mincing pieces of 

 the ripe ovary and testes separately in water and then by mixing the two 

 fluids. 



Scott considers that fertilization takes place within the female. He 

 made some experiments to demonstrate this. At Piel the first eggs that 

 were spawned were not fertilized. After the first discharge of sperms, 

 however, the eggs were always fertilized. 



Garner thought that the mussel was hermaphrodite and that the eggs 

 were already fertilized when discharged. 



The Mature Egg. 



When the eggs are present on the bottom of the aquarium in quantity, 

 they show a pink colour. In the case of the Faroe mussel the colour of 

 the eggs was hid by some colourless tissue that was extruded with them. 



Wilson describes the ripe egg thus: — "The mature egg after extrusion 

 is quite spherical and very opaque, on account of the large development 

 of deutoplasmic (vitelline) granules. The vitellus appears greenish-brown 

 by transmitted light. The egg membrane is distinct. The nucleus and 

 nucleolus are not visible in a normal mature egg, a less opaque region 

 marking their position. The hyaline investment noticeable around the 

 iiitra-foUicular egg forms a broad, very translucent sheath to the ripe 

 extruded eggs. It is presumably adherent to mature eggs naturally 

 extruded — certainly so in the case of artificially-liberated ones. Many 

 spermatozoa wriggle into it and cause the egg to rotate. No micropyle 

 has been observed in it. If the vitelline membrane be ruptured the 

 nucleus escapes as a spherical body bounded by a definite membrane and 

 enclosing a nucleolus." 



The eggs spawned by the Faroe mussel showed the following characters 

 (fig 6). They were, to the naked eye, greyish in colour with a faint 

 amber tinge. They were round in some cases, usually oval, measuring 

 •07 by 06: "07. One was as long as -Imm. 



The yolk is seen to be broken up into several large corpuscles or 

 divisions, and at the periphery there are minute corpuscles similar to 

 those that are found in the developing egg. 



The zona is wrinkled, corrugated on the outside, and raised into little 

 papillae. The corrugations seem to be due to the yolk being in large 

 corpuscles which boss the zona. It has two definite layers, and in 

 addition some eggs show what looks like a thin membrane on the yolk. 

 The latter may be simply the sharp outer edge of the protoplasm. 

 There are little protoplasmic thread-like projections from parts of the 

 zona, possibly from pores (fig. 8). In one case a pitted condition of 

 the zona was thought to be made out. 



A faint indication of the nucleus was made out in some, and surround- 

 ing it large yolk corpuscles or divisions composed of the minute yolk 

 corpuscles. The large nucleus seems to be broken up more or less into 

 large round bodies. 



On pressing the egg by means of the cover-glass, the contents do not 

 flow out so readily as in an egg dissected out of the mantle in a ripening 

 ovary. In the latter case nearly every egg exhibits the loss of some of 

 its yolk contents. 



One of the naturally spawned eggs, when pressed lightly, took a pear 

 shape with a conical end. This seemed to be at the micropyle, 

 and to be formed by the forcing of the egg contents to project through it. 



