BATE OF GROWTH OF OSTREA VIROINIOA. 725 



any such 1 raiments nicely, I find n small strainer of coarse bolting or cheese cloth to be very 



Convenient. 



In the foregoing description wo have described the method of obtaining the spawn only from 

 the side of the animal exposed in opening the shell. A little exi>erience will enable one to lift up 

 the head end of the animal and throw it back over the great adductor muscle, expose the opening 

 of the reproductive organ on the left side, or whatever the case may be, and also express the 

 spawn from that side, thus as effectually obtaining all of the ripe eggs or milt as is possible in the 

 process <>t taking the same from fishes. 



It is remarkable to note the success attending this method, since almost every egg is perfect 

 and uninjured, the percentage of ova which are impregnated is much larger than by the old 

 method, reaching, I should say, quite ninety per cent, of all that are takeii when the products are 

 perfectly ripe. It is also found that the products are not so readily removed by my process if they 

 are not perfectly mature, which is also to a certain extent a safeguard against getting poor or 

 immature spawn. In the course of an hour after the products of the two sexes have beeu mingled 

 together it will be found that nearly every egg has assumed a globular form, has extruded a polar 

 cell, lost the distinct germinative vesicle and spot in the center, and begun to develop. 



It is noteworthy that our practice as herein described has completely vindicated the state- 

 ment made by the distinguished French anatomist and embryologist, M. Lacaze-Duthiers, that 

 there is but a single generative opening on each side of the visceral mass of the Oyster, and that, 

 as we have stated, it is found to open just below the great adductor muscle. 



We have also discovered, since the foregoing was written, that the use of an excessive amount 

 of milt is of no advantage. The water in which the eggs are to be impregnated only requires to 

 be rendered slightly milky : a very few drops of good milt is sufficient to make the impregnation 

 a success. Too much milt causes the eggs to be covered by too large a number of spermatozoa ; 

 thousands more than are required if too much is used. These superfluous spermatozoa simply 

 become the cause of a putrescent action which is injurious to the healthy development of the eggs. 

 A drop of milt to twenty drops of eggs is quite sufficient. 



Immediately after the ova have been fertilized it is best to put them into clean sea-water at 

 once, using water of the same density as that in which the adults grew. If the attempt is made 

 to impregnate the eggs in water much denser than that in which the adults lived, it is probablo 

 that the milt will be killed at once. This singular fact, which was accidentally discovered by 

 Colonel McDonald and myself, shows how very careful we should be to take into consideration 

 every variation in the conditions affecting a biological experiment. If sufficient water is used no 

 trouble will be experienced from the pollution of the water by dangerous micro-organisms which 

 are able to destroy the oyster embryos. From fifty to two hundred volumes of fresh, clean water 

 may be added to the volume in which the eggs were first fertilized. This may be added gradually 

 during the first twenty- four hours, so as to assist aeration and prevent the suffocation of the 

 embryos. 



214. RATE OF GROWTH OF OSTREA VIRGHflCA. 



SIZE OP THE EGG. The egg of the American Oyster, according to Brooks, is approximately 

 3^ inch, being very nearly perfectly spherical after the extrusion of the polar or direction cells 

 (RichtungsbUischen of the German embrvologist*). This accords with what the writer has observed 

 in our species, and in the Portuguese Oy-ter, probably 0. angulata Lain., the size of the egg 

 appears to be about the same, judging from specimens of the latter examined by me in March 

 last. Judging from the figures and the stated amplifications given in the papers of M. Davainc, 



