214 THE FISHERIES OF THE UNITED STATES. 



This layer is seen in the section to lie pushed in a little to-n-ard the upper layer, so that the lower surface of the disk-shaped embryo 

 is not flat, but very slightly concave. This concavity is destined to grow deeper until its edges almost meet, and it is the rndinu'ntary 

 di"-estive cavity. A very short time after this stage has Ijeen reached, and usually within from two to four hours after the eggs were 

 fertilized, the embryo undergoes a great change of shape, and assumes the form whicli is shown in three difi'erent views in Figures 32, 33, 

 34, and 35. 



A circular tuft of long hairs, or cilia, has now made its appearance at what is thus marked as the anterior end of the body, and .as soon 

 as these hairs are formed they begin to swing backward and forward in such a wiiy as to constitute a swimming organ, which rows the 

 little animal up from tlie bottom to the surface of the water, where it swims around very actively by the aid of its cilia. This stage of 

 development, Figure 32, which is of short duration, is of great importance in raising the young oysters, for it is the time when they can 

 best be siphoned off into a separate vessel and freed froui the danger of being killed by the decay of any eggs which may fail to develop. 

 On one siuface of the body at this stage, the dorsal surface, there is a well-uiarked groove, and when a specimen is found in a proper 

 position for examinati(m, the opening into the digestive tr.act is found at the bottom of this groove. Figure 33 is a sectional view of such 

 an embryo. It is seen to consist of a central cavity, the digestive cavity, which opens externally on the dorsal surface of the body by a 

 small orifice, the primitivl^ mouth, and which is surrounded at all points, exo ept at the mouth, by a w.all which is distinct from the outer 

 wall of the body. Around th(! primitive mDUth these two layers are continuous with each other. 



The way in which this cavity, with its wall and external opeuiug, has bcjn formed, will be understood l)y a comparison of Figure 

 33 with Figure 28. The layer which is below in Figure ^8 has been pushed upward in such away as to convert it into a long tul)e, and 

 at the same time the outer layer has grown downward and inward around it, and has thus constricted the opening. The layer of cells 

 which is below in Figure 28 thus becomes converted into the walls of the digestive tract, and the space which is outside and below the 

 embryo, in Figure 28, becomes converted into an inclosed digestive cavity, which opens externally by the primitive mouth. 



This stage of development, in which the embryo consists of two layers, an inner layer surrounding a cavity which opens externally 

 by a mouth-like opening, and an outer layer, which is coutinuous with the inner around the margins of the opening, is of very fre(|nent 

 occurrence, and it has been found, with moditicatious, in the most widely separated groups of animals, such as the starlish, the oyster, 

 and the frog, and some representatives of all the larger groups of animals, except the Protozoa, appeiir to pass during tlicir development 

 through a form which may be regiirded as a more or less considerable modification of that presented by our oyster-embryo. This stage of 

 development is known as the qantrnla stage. 



Certain full-grown animals, such. as the fresh-water hydra and some sponges, are little more than modified gastrulas. The body is a 

 simple vase, with an opening at one end communicating with a digestive cavity, the wall of which is formed by a layer of cells, which is 

 continuous around the opening with a second layer, which forms the outer wall of the body. This fact, together with the fact that 

 animals of the most widely separated groups pass through a gastrula stage of development, has led certain naturalists to a generalization, 

 which is known as the " gastrula theory". This theory or hypothesis, is that all animals, except the Proio:oa, are more or less direct 

 descendants of one common but very remote ancestral form, whose body consisted of a simple two-walled vase, with a central digestive 

 cavity opening externally at one end of the body. 



The edges of the primitive mouth of the oyster continue to approach each other, and finally meet and unite, thus closing up the 

 opening, as shown in Figure 36, aad leaving the digestive tract without any comuiunicatiou with the outside of the body, and entirely 

 surrounded by the outer layer. The embryo shown in Figures 32 and 36 are represented with the dorsal surface below, in order to facilitate 

 compariscm with the adult, but in Figirre 37, and most of the following figures, the dorsal surface is uppermost, for more ready comparison 

 \fith the adult. The furrow in which the primitive mouth was placed still persists, and soon a small irregular i)late makes its appearance 

 at each end of it. These little plates are the two valves of the shell, and in the oyster they are separated from each other from the first, 

 and make their aitpearance independently. 



Soon after they make their appearance, the embryos cease to crowd to the surface of the water, and sink to various depths, although 

 they continue to swim actively in all directions, and may still be found occasionally close to the surface. The region of the body which 

 carries the cilia now becomes sh-arply defined, as a circular projecting pad, the velum, and this is present and is the organ of locomotion 

 at a much later stage of development. It is shown at the right side of the figure in Figure 37, and in Figure 45 it is seen in surface view, 

 drawn in between the shells, and with its cilia folded down and at rest, as they are seen when the little oyster lies upon the bottom. 



The two shells grow rapidly and soon become quite regular in outline, as shown in Figures 37 and 44, but for some time they, are 

 much smaller than the body, which projects from between their edges around their whole circumference, except along a short area, the 

 area of the hinge, upon the dorsal surface, where the two valves are in contact. 



The two shells continue to grow at their edges, and soon become large enough to cover up and project a little beyond the surface of 

 the body, as shown in Figure 44, and at the same time muscular fibers make their appearance and are so arranged that they can draw the 

 edge of the body and the velum in between the edges of the shell, in the manner shown in Figure 45. In this way that surface of the 

 body which lines the shell becomes converted into the two lobes of the mantle, and between them a mantle cavity is formed, into which 

 the velum can be drawn when the animal is at rest. While these changes have been going on over the outer surface of the body, other 

 important internal modifications have taken place. We left the digestive tract at the stage shown in Figure 36, without any communi- 

 cation with the exterior. 



Soon the outer wall of the body becomes pushed inward, to form the true month, at a point (Figure 37) which is upon the ventral 

 surface, and almost directly opposite the point where the primitive mouth was situated at an earlier stage. The digestive cavity now 

 becomes greatly enlarged, and cilia make their .appearance upon its walls, the mouth becomes connected with the ch.amber which is 

 thus formed, and which becomes the stomach, and minute jiarticles of food are drawn in by the cilia, .and can now be seen inside the 

 stomach, where the vibr.ation of the cilia keep them in constant motion. Up to this time the animal has developed without growing, and 

 at the stage shown in Figure 36 it is scarcely larger than the unfertilized egg, but it now begins to increase in size. The stages shown in 

 Figures 44 an<l 45 agree pretty closely with the figures which European embrvologists give of the oyster-embryo at the time when it 

 escapes from the mantle chamber of its parent. The American oyster reaches this stage in from twenty-four hours to six days after the 

 egg is fertilized ; the rate of development being determined mainly by the temperature of the water. 



Soon after the mantle has become connected witli the stomach, this becomes united to the body wall at another point a little iM'hind 

 the mantle, and a second oiiening, the anus, is formed. The tract which connects the anus with the stomach lengthens and forms the 

 intestine, and, soon after, the sides of the stomach become folded oft' to form the two halves of the liver, as shown in Figure 44. 



Various muscular fibers now m.ake their appearance within the body, an<l the animal assumes the form shown in Figures 44 and 45. 

 All my attempts to get later stages than these failed, through my in.ability to find any way to ch.ange the water without losing the 

 young oyster, and I am therefore uuable to describe the manner in which the swimmingembryo becomes converted into the adult, Init I 

 hope that this gap will be filled, either by future observations of my own or by those of some other embryologist. 



