[21] FISHEKIES OF THE UNITED STATES. 205 



to remove it after the embryos have begun to swim, without loosing 

 them as well. 



"After a final stirring, the beaker should be allowed to stand for about 

 five minutes, to allow the eggs to settle to the bottom, and the fluid above 

 them should then be drawn off through a siphon, reaching nearly but 

 not quite down to the eggs. A fresh supply of sea-water should then 

 be added, and the eggs stirred and allowed to settle, and the water 

 drawn off as before; and this should be repeated until the water, after 

 the eggs have settled to the bottom, remains clear. 



"The beaker may now be set aside where it will not be exposed to 

 sudden changes of temperature, and the eggs will require no further 

 attention until the embryos begin to swim, which will be in from two 

 to six hours, according to the temperature. The little oysters must of 

 course be supplied with fresh sea- water from time to time during their 

 development, and as they are so small that the water cannot be drawn 

 off after they begin to swim, they must be supplied with fresh water 

 by transferring them from time to time to larger and larger beakers. 

 In two hours or so after the eggs are fertilized the embryos begin to 

 swim, and crowd to the surface of the water in great numbers, and form 

 a thin stratum close to the surface. This layer of embryos may be 

 carefully siphoned off into a very small beaker, and a little fresh sea- 

 water added. In an hour or so there will be a new layer of embryos 

 at the surface of beaker No. 1, and these should also be siphoned into 

 No. 2, and this should be repeated as long as the embryos continue to 

 rise to the surface of the first beaker. Every five or six hours a little 

 fresh sea-water should be poured from a height of a foot or more into 

 beaker No. 2, until it is filled. The contents should then be poured into 

 a larger beaker, and sea- water added four or five times a day as before. 

 In this way the embryos may be kept alive for a week, although they 

 have by this time got into such a large vessel that it is almost impos- 

 sible to find any of them for microscopic examination. * 



"THE DEVELOPMENT OF THE EGGS. , 



"I will now attempt a brief, popular account of the changes through 

 which the fertilized egg is gradually converted into the complex body 

 of the adult oyster. 



"The body of the oyster, like that of all animals, except the very 

 simplest, is made up of organs, such as the heart, digestive organs, 

 gills, and reproductive organs; and these organs are, at some period in 

 the life of the oyster, made up of microscopic cells. The eggs shown 

 in Figs. 49 and 53 will answer to illustrate the character of the cells 

 which compose the body; each of these consists of a layer of protoplasm 

 around a central nucleus, which, in the egg, is a large, circular, trans- 

 parent body known as the germinative vesicle. Each cell of the body 

 is able to absorb food, to grow and to multiply by division, and thus 

 to contribute to the growth of the organ of which it forms a part. The 



