Yet each has a little mass of protoplasm, a nucleus, andr 

 all the other parts that a cell is entitled to have. 

 Furthermore, each one of the 2X chromosomes in the 

 nucleus made up of the union of male and female nuclei 

 splits with every division, and thus every nucleus has still 

 2X chromosomes. 



The first difference in these young cells showed at the 

 eight-celled stage when we had four small and four large 

 cells. At this time we know that the small cells are to 

 make the outer part of the skin, the nervous system, and 

 the sensitive parts of the animal generally. The larger 

 cells will give rise to the digestive tract and its glands. 

 These cells then are different, although they are all 

 descended from the same cell at the beginning. This is 

 called differentiation. The cells, from this time on, 

 differentiate more and more. Some of them become nerve 

 cells, some become bone, others become gland cells, 

 muscle cells, connective tissue, blood cells, pigment cells, 

 and so forth. 



In three or four days changes enough have occurred to 

 enable us to see at a glance which end is to be head and 

 which tail; which side is to be back and which belly; 

 which right and which left. In a few days more, the 

 young tadpole breaks out of the envelop of gelatin and we 

 say it is hatched. From this point on it leads a different 

 life, because its environment is greatly changed. 



5. The Life of the Tadpole. As the young frog, which 

 is called a tadpole, develops, it has organs suiting it to life 

 in the water. It has a soft skin that would dry up in a 

 few minutes in the air, it has gills by which it takes its 

 oxygen from the water, it has a fin-like tail by which it 

 swims freely, it has no legs that would enable it to walk. 

 In a word, it is a water-living, fish-like creature with no 

 adaptations whatever for an air and land life. During a 

 period varying from three or four months to two years, 

 depending on the species and the conditions of life, this 

 tadpole may continue. During this time it eats vegetable 

 matter. The tadpoles of the large species like the bull- 

 frog may become three or four inches in length. 



6. The Great Change. If the tadpole stopped here, his 

 history, perhaps, would be as interesting as most others. 

 He might be considered a strange kind of fish. But he 

 does not stop here. His later behavior is one of the 

 surprises of nature. Usually during the same summer in 

 which he hatched, he begins to undergo some changes that 

 almost make him over into a new animal. These are both 

 inside and outside changes. On the outside he begins to 

 grow legs, first the hind pair where the tail joins the body, 

 and later a front pair close to the head. During this time 

 the tail is gradually being absorbed by the blood corpuscles 

 that circulate through it. By means of the blood this 

 material is turned over to the growing animal very much 

 as though he had eaten his own tail and digested it. The 

 tail finally disappears entirely. 



The inside changes are even more wonderful. The 

 intestine, coiled like a watch spring, in which he digested 

 vegetable food, becomes more straight, and becomes suited 

 to animal food. The gills by which he got oxygen from 

 the water gradually disappear, while deeper in his body 

 is developing a pair of lungs into which he takes air 

 through the nostrils. The blood vessels from the heart 

 change so they can take proper care of this new kind of 

 supply of oxygen. The frog doesn't take a vacation during 

 this time. He has to capture food, and must keep his wits 

 about him every minute so that some otner animal will not 

 capture him. It is really a remarkable performance. It is 

 as though a gasoline launch were gradually changed into 

 an electric automobile, which could run either on land 



or in water, and during the whole of the transformation 

 was a perfectly usable vehicle! 



CHAPTER SEVENTEEN. 

 THE BIRDS. 



1. When the Chicken Leaves the Egg. When a young 

 chicken, or other bird, comes out of the egg, we are 

 disposed to take him as he is and ask no questions about 

 his past. At this time the young bird is enough like its 

 parents for us to recognize it as a bird. To be sure, it 

 makes some changes after it hatches, but it has all the 

 organs of the adult, and it lives in the same general way 

 that it does when it grows up. It is not .very hard to 

 imagine that a young chick plus proper food should in due 

 time become a laying hen or a fighting cock. After 

 hatching it passes through no such changes as we saw in 

 the passage of the tadpole into the frog. 



But the events back of this appearance of the bird from 

 the egg make a most remarkable chapter in its history. 

 Indeed many of the things that happen within the shell 

 are not unlike the passage of the tadpole into the frog. 



2. The Character of the Egg. The egg of the bird has 

 a limy shell about it, and inside that is a tough membrane. 

 Just within this membrane is the sticky "white" of the egg. 

 Not one of these things is a part of the real egg. All of 

 them were added after the egg was formed. The real egg 

 is the part we call the yellow, or yolk. 



Most of us have seen in the body of an old hen, being 

 prepared for the table, the ovary with a number of yolks 

 of different sizes, like little potatoes around a potato 

 plant. These are of different ages, and one or two may be 

 about as large as the yolk in an ordinary egg. The larger 

 ones are about ready to escape from the ovary and start 

 to the outside world. When the egg (or yolk) is ready 

 it passes from the ovary into the body cavity and enters 

 the large opening of the oviduct, or egg tube, which- 

 carries it to the outside world. As it goes down the 

 oviduct the wall of this tube secretes the albumen (white) 

 all round the yolk. It goes gradually twisting down the 

 oviduct and finally comes to rest at a point where the 

 shells are formed around it. Soon afterward the hen lays 

 the egg. 



3. Fertilization and Development. It must be clear 

 that fertilization of an egg like this could not take place 

 as it does in the frog. A sperm could not possibly work 

 its way through the shell or through the sticky albumen. 

 If the egg is to be fertilized, it is necessary for the sperm 

 to unite with it early in its course through the oviduct, 

 and before any of these materials are added. This means 

 that the sperm cells must be inserted in the outer opening 

 of the tube and then ascend to the upper and inner part 

 of it where the egg enters. This is just what happens. 

 When the male and female bird mate, or copulate, the 

 sperms are discharged in large numbers into the cloaca, 

 or opening of the digestive tract, from which they make 

 their way upward into the oviduct. Only one is necessary 

 to fertilize each egg. The others die. 



After fertilization it requires the fertilized egg, or 

 embryo, several hours to pass down the oviduct, get its 

 shells, and escape. During this time the nucleus, or 

 protoplasm, of the young embryo divides a number of 

 times, so that when it is laid it is not really an egg. It is 



