720 NATURAL HISTORY OF AQUATIC ANIMALS. 



ripe female may be washed out of the ovary into a beaker of sea-water, and, as they are heavier 

 than the sea-water, they soon sink to the bottom, and the eggs of a medium-sized female will 

 cover the bottom of a beaker two inches in diameter with a layer of eggs one-twentieth of an inch 

 deep. The area of the bottom of a beaker two inches in diameter is little more than three square 

 inches, and a layer of eggs one-twentieth of an inch deep, covering three square inches, is equal to 

 one three twentieths of an inch deep and two square, and as a single layer of eggs is one five- 

 hundredth of an inch thick, a layer three-twentieths of an inch thick will contain seventy-five 

 layers of eggs, with 250,000 eggs in each layer, or 18,750,000 eggs. It is difficult to get the eggs 

 perfectly pure, and if we allow one-half for foreign matter and errors of measurement, and for 

 imperfect contact between the eggs, we shall have more than nine millions as the number of eggs 

 laid by an Oyster of average size, a number which is probably less than the true number. 



" Mobius estimates the number of eggs laid by an average European Oyster at 1,012,925, or only 

 one-ninth the number laid by an ordinary American Oyster; but the American Oyster is very much 

 larger than the European, while its eggs are less than one-third as large; so the want of agreement 

 between these estimates does not indicate that either of them is correct.' Another estimate of 

 the number of eggs laid by the European Oyster is given by Eyton (' History of the Oyster and 

 Oyster Fisheries,' by T. 0. Eyton, London, 1858). He says, p. 24, that there are about. 1,800,000, 

 and therefore agrees pretty closely with Mobius. 



"An unusually large American Oyster will yield nearly a cubic inch of eggs, and if these were 

 all in absolute contact with each other, and there were no portions of the ovaries or other organs 

 mixed with them, the cubic inch would contain 500 3 , or 125,000,000. Dividing this, as before, by 

 two, to allow for foreign matter, interspaces, and errors of measurement, we have about 60,000,000 

 as the possible number of eggs from a single Oyster. 



" Although each male contains enough fluid to fertilize the eggs of several females, there does 

 not seem to be much difference in the number of individuals of the two sexes. When a dozen 

 Oysters are opened and examined, there may be five or six ripe females and no males, but in 

 another case a dozen Oysters may furnish several ripe males but no females, and in the long run 

 the sexes seem to be about equally numerous. Oystermeu believe that the male may be dis- 

 tinguished from the female by certain characteristics, such as the presence of black pigment 

 in the mantle, but microscopic examination shows that these marks have no such meaning, and 

 that there are no differences between the sexes except the microscopic ones. It is not necessary 

 to use the microscope in every case, however, for a little experience will enable a sharp observer 

 to recognize a ripe female without the microscope. If a little of the milky fluid from the ovary of 

 a female with ripe or nearly ripe eggs be taken upon the point of a clean, bright knife-blade and 

 allowed to flow over it in a thin film, a sharp eye can barely detect the eggs as white dots, while 

 the male fluid appears perfectly homogeneous under the same circumstances, as do the contents of 

 the ovary of an immature female, or one which has finished spawning. When the eggs are mixed 

 with a drop of water they can be diffused through it without difficulty, while the male fluid is 

 more adhesive and difficult to mix with the water. By these indications I was able in nearly 

 every case to judge of the sex of the Oyster before I had made use of the microscope. 2 



"During my investigations I submitted more than a thousand Oysters to microscopic 

 examination. My studies were carried on during the breeding season, and I did not find a single 



'Miibios' measurement, from .15 to .18 millimeter, is given (Austern und Austernwirthschaft, 1877) as the diameter, 

 not of the egg, but of the embryo, but his figures show that the European Oyster, like the American, does not grow much 

 during the early stages of development, but remains of about the same size as the egg. 



1 W. K. BROOKS: op. cit., pp. 13-15. 



