. Adventures with animals and plants. Biology. 548 The Earth mid Its Inhabitants Change unit x ^^^^^^^F^^^^^^l ? ^^^^^H'^^I^^^H ^^^^^1 ?ttLj^Hm 9|P^^9 ^^^^1 ^^^^^BE^niv^" i^Cd^^^B ^BTfj^^JS^ J, y F^ Wm^ ?^/? Wm' fl Hk* -sr «« 1 m^^ ' % J J ^m ?. Fig. 499 No?(? the smiilarhies in the skeletons: turtle at left, frog in center, and cat at right. All three are vertebrates. To what class does each belong? Are the skeletons exactly alike? (ward's natural science establishment) How can resemblances in structure be explained? The study of the structure of organisms is called anatoviy. The com- pa
. Adventures with animals and plants. Biology. 548 The Earth mid Its Inhabitants Change unit x ^^^^^^^F^^^^^^l ? ^^^^^H'^^I^^^H ^^^^^1 ?ttLj^Hm 9|P^^9 ^^^^1 ^^^^^BE^niv^" i^Cd^^^B ^BTfj^^JS^ J, y F^ Wm^ ?^/? Wm' fl Hk* -sr «« 1 m^^ ' % J J ^m ?. Fig. 499 No?(? the smiilarhies in the skeletons: turtle at left, frog in center, and cat at right. All three are vertebrates. To what class does each belong? Are the skeletons exactly alike? (ward's natural science establishment) How can resemblances in structure be explained? The study of the structure of organisms is called anatoviy. The com- parison of the structure of different ani- mals is a whole division of biology known as cojnparative anatoviy. Students of comparative anatomy have long been puzzled by the resemblances in the inter- nal structure of animals that are so dif- ferent in outward appearance. From their study of fossils they can now ex- plain these resemblances. Fossils seem to show that all vertebrates are related more or less distantly. If they are related one would expect them to have similarities. The study of fossils helps further to explain why a particular animal (or plant) should be more like one form and less like another. A fish resembles a frog more than a cat. According to the fossils, frogs appeared soon after the first fish. Mammals appeared much later. This makes it seem likely that fish are more closely related to frogs than to cats. Similarity in embryos. One of the most interesting things about vertebrates is that the embryos of all of them are so much alike. In all of them there is cleav- age of a fertilized t^<s,^ followed by the hollow ball (blastula) and cup (gastrula) stages. Then differentiation occurs form- ing the various tissues and organs. Even after much differentiation the embryos are much alike, no matter whether the animal will finally be a fish, an amphib- ian, a reptile, a bird, or a mammal. Figure 501 shows that it is difficult to tell one from another. You will
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