234 BULLETIN OF THE UNITED -STATES FISH COMMISSION. 



queiitly iu January fouud in tlie glands producing- oggs and sperm 

 small but well-formed eggs and spermatic animalcula, which had only 

 to be brougiit in contact with the sea-water to produce animation and 

 vibration. 



From April and May the first milky oysters are found. The term 

 "milky" is employed because the spawning products, when emitted, 

 present the api^earauce of thick milk. But when this so-called "milk" 

 IS examined under the microscope it will be found to consist partly of 

 freshly fecundated eggs, partly of eggs which have already split, and 

 of embryos recently hatched. As these develop, the whitish color of 

 the spawn changes and it turns gray; and when they approach their 

 complete transformation and full maturity, their color gradually turns 

 from a dark gray to black. 



The number of embryos produced by a single oyster during the year 

 is estimated at from 1,200,000 to 1,500,000.* The embryos are sheltered 

 iu the folds of the mantle of the mother oyster for a period whose 

 length has not yet been ascertained, and find in the mucilaginous liquid 

 in which they live the nutritive elements which thev need in order to 

 reach the period when their roving or pelagic life commences. 



Some naturalists have asserted that the oyster can spawn twice in 

 one year. What has given rise to this supposition is the circumstance 

 that frequently during the spawning season there are two well-defined 

 emissions, the first about the beginning of June, the second towards 

 the end of August. 



The following experiment shows that there is no good reason lor this 

 opinion, and that it is not founded on fncts. Proposing to follow up 

 the incubation of the eggs and the transformation of the embryos in 

 the interior of the shell, we perforated the upper valve of several 

 mother oysters. This opening, which was made directly opposite the 

 incubatory cavity, was corked up, so that no water could enter the 

 oyster, and we could at any time take the observations which the ex- 

 periment required. As regards the principal object of the inquiry, tli*i 

 data gathered by us were not very ])recisc, as most of the oysters wdiich 

 had been perforated had emitted their spawn, either at the time when 

 the operation of ])erforating the shell took place, or one, two, or four 

 days afterwards; l)ut as regards the final development of the genital 

 gland, we could see this gland re-form, grow, and become covered w^th 

 fat; and although the speciuiens examined by us had produced embryos 

 from the beginning of the spawning season, we never noticed that they 

 were in condition to emit others at the end of the season. 



* M. Gerbe, tlic distinguished co- worker of M. Costo, who has taken so hxrgo and 

 active a share iu the work of tlie last-named naturalist, has found that oysters may 

 lie considered adult, when they have reached tiu^ age of one year. Oysters of this 

 age, l>y reason of their size, are not as fecund as those of which W(> are about to 

 speak. The.se measure H to 10 centimeters [aliout '.U inclu-s], a size which is usually 

 not reached until they are three to four yeara old. 



