548 



The Earth mid Its Inhabitants Change unit x 



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 note that not only the fish 

 and frog, but the land-living forms as 

 well, have gill slits in their early stages. 

 Even the human embrv^o in its early 

 stages has gill slits. For a considerable 

 time, too, the human embr\''o has a tail. 

 This region stops growing at an early 

 stage, and as the embr\^o gets larger the 

 tail becomes less and less evident. A few 

 small bones remain as the coccyx (cock'- 

 six). Also, in the coui-se of the develop- 

 ment of the human embiyo there is a 

 time when its heart is like that of the 

 lower vertebrates; only gradually does it 

 develop the four chambers. It is apparent 

 that the human embryo in its early stages 



