The Oyster: A Popular Summary of a Scientific Study

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Johns Hopkins Press, 1891 - Oyster-culture - 230 pages
 

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Page 52 - ... 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 chamber which is thus formed, and which becomes the stomach, and minute particles of food are drawn in by the cilia, and can now be seen inside the stomach, where the vibration of the cilia keep them in constant motion. Up to this...
Page 52 - ... the rate of development being determined mainly by the temperature of the water. Soon after the mantle has become connected with the stomach this becomes united to the body wall at another point a little behind the mantle, and a second opening, 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 off to form the two halves of the liver, as shown in plate ix, fig.
Page 52 - 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 plate vni, fig.
Page 46 - Recent investigations tend to show that while these changes are taking place, one of the male cells penetrates the protoplasm of the egg and unites with the germinative vesicle, which does not disappear, but divides into two parts, one of which is pushed out of the egg and becomes the polar globule, while...
Page 47 - Fig. 6, were split in the plane of the paper, we should have what is to become one half of the body in one part and the other half in the other. The single spherule at the small end of the pear is to give rise to the cells of the digestive tract of the adult, and to those organs which are to be derived from it, while the two spherules at the small end...
Page 47 - This layer is seen in the section to be pushed in a little toward 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 rudimentary digestive cavity. A very short time after this stage has been reached, and usually within from two to four hours after the eggs-were fertilized, the embryo undergoes a great change of shape.
Page 53 - They are so perfectly defenseless, and so crowded together close to the surface, that a small fish, swimming along with open mouth, might easily swallow, in a few mouthfuls, a number equal to a year's catch. They are also exposed to the weather, and a sudden cold wind or fall in temperature, such as occurred several times during our experiments, killed every embryo. The number which are destroyed by cold rains and winds must be very great indeed. As soon as they are safely past this stage, and scatter...
Page 47 - ... is now made of one large mass and two slightly smaller ones, with the polar globule between them. The later history of the egg shows that at this early stage the egg is not perfectly homogeneous, but that the protoplasm which is to give rise to certain organs of the body has separated from that which is to give rise to others.
Page 52 - ... 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 Fig.
Page 48 - Fig. 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 primitive mouth, and which is surrounded at all points, except at the mouth, by a wall which is distinct from the outer wall of the body. Around the primitive mouth these two layers are continuous with each other. The way in which this cavity, with its wall and external opening, has been formed, will be understood...

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