212 THE FISHERIES OF THE UNITED STATES. 



very careful watching will show that each of them has a lively dancing motion, and examination with a power of 

 500 diameters will show that each of them is tadpole-shaped (Figure 50), and consists of a small, oval, sharply 

 defined 'head', and a long, delicate 'tail', by the lashing of which the dancing is produced." These are the 

 spermatozoa, or "male cells", whose union with the eggs or ova of the female is necessary to the iertilization of the 

 latter, and thereon sequent hatching of living oysters. 

 Again quoting from Dr. Brooks : 



The number of male cells vbicli a single male ■will yield is great beyond all power of expression, but the number of eggs which 

 an average female will furnish may be estimated with sufficient exactness. A single ripe egg measures about one five-hundredth 

 of an inch in diameter, or five hundred laid iu a row, touching each other, would make one inch ; and a square inch would contain 

 five hundred such rows, or 500 by 500^2.'i0,000 eggs. Nearly all the eggs of a perfectlj' ripe female may be washed out of the ovary 

 into a beaker of sea-water, and as tliey 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 oue-twentieth of an inch deep. The area of the 

 bottom of a beaker two inches in diameter is a 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, aud as a single layer of eggs is one five-, 

 hundredth of ;iu inch thick, a layer tliree-twentic^ths of an inch thick will contain seventy-five layers of eggs, with 250,000 eggs in each 

 liiyer, or 1^,750,000 eggs. It is diliScult to get the eggs perfectly pure, and if we allow one-half for foreign matter aud errors of measurement, 

 and for imperfect coutact 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 th.in 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 lairl by an 

 ordinary American oyster, but the American oyster is very much larger than the Knropeau, 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 incorrect.* Another estimate of the number of 

 eggs laid by the European oyster is given by Eytou {Histoi-ij of the Oiister and Oijuter Fisheries, by T. C. Eytou, London, 1858). He says, 

 p. 24, that there are about 1,800,000, and therefore agrees pretty closely with Miibins. 



An unusually large American oyster will yield neiirly 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'', 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 numb(-r 

 of eggs from a single oyster. 



Although each m.ile contains enough fluid to fertilize the eggs of sevei-al females, there does not seem to be much difi'erence in the 

 numlx'r of individuals of the two sexes. When a dozen oysteis are oijened and examined, there may be five or six ripe females and no 

 males, but in another case a dozen oysters m.ay furnish several ripe males but no females, and in the long run the sexes seem to be about 

 equally numerous. Oystermen believe that the male may be distinguished 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 them are no 

 differences between the sexes except the microscopic ones. It is not necesf-ary 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 au immature female, or one which has fluishcd spawning. When the eggs are mixed with a drop of watc^-, 

 they can be diffused through it without diflicult-y, 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. 



Sexual Differences. — This question of sex, and the condition nnder which impregnation takes place in 

 oysters, must next be considered. To tliis question Dr. Brooks devoted himself with special attention. 



About all the published information upon the subject referred to the European species, and stated that, by means 

 of spermatozoa, discharged into the water by neighboring oysters, and sucked within the shell, the eggs are fertilized 

 inside the body of the parent, aud that the young are carried inside the i)areut shell until they are quite well 

 advanced in development and provided with shells of their own; that they swim about after they are discharged 

 from the parent until they find a ])lace to attach themselves, but that they undergo no change of structure between 

 the time wlien they leav'e the parent and tlio time when they becotue fixed. JMisled by these statements, Dr. Brooks 

 opened many oysters during the summer of 1S78, and carefully examined the contents of the gills and mantle 

 chambers, but found no young oysters. " I concluded," he says, "that the time during which the young are carried 

 by the ])areut must be so short that I had missed it, and I entered upon the work this season with the determination 

 to examine adult oysters every day, through tlie breeding-season, in search of young, aud at the same time to try 

 to raise the young for myself by artificially fertilizing the eggs after I had removed them from the body of the 

 parent." The result of a diligent practice of the first of these resolutions surprised him. In the first place he 

 proved anew the generally admitted doctrine, that oysters are not hermaphroditic ; in other words, that each oyster 

 is, at the breeding-season, either a male or a female. He writes : 



During my investigations I su1)mitted morci than a tliousiind oysters to niiscroscopie ex.amiuation. My studies were carried on during 

 the brec<liug-seiison, and I did not find a single hermaphrodite. The male cells are so small coinjiared with the eggs, that it would bo 

 im])ossible to state that a mass of eggs taken from tlu^ ovary contained no spermatozoa, although they could not escape detection if they 

 were at all .abundant. 



On the other hand, a single egg in tlu^ field of tlie microscope, iu a drop of male fluid, would be very conspicuous, mid ciuild not- 

 escape detection ; and the fact that not a single case of this kind occurred, is stillicient to establish the distinctness of the s(--xes at the 

 breeding-season. 



* Miihius' measurement, from 0.15 to 0.18 millimeters, is given (Austern und Austern-wirtschaft, 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, hut remains of about the same size as the egg. 



