a young chicken of, perhaps, several hundred cells. It 

 does not at this time look the least like a chicken. It is 

 just a little round spot which one sees on top of the yolk 

 if an egg is broken and put in a saucer. 



4. Development and Hatching. Such an egg as this, 

 which is really a young chicken, will keep its life for a 

 while if the egg is put aside, but not for very long. If it is 

 left long the young chick dies, and the egg begins to 

 decompose. If, however, the egg when first laid is put in 

 a constant, warm temperature, the cell division continues, 

 and the young chick which was at first just a little spot of 

 cells on one side of the yolk, grows all round the yolk. It 

 develops a heart, and blood vessels that run out over the 

 yolk and collect food from it and bring this back to the 

 growing parts. Thus when the egg is warmed by the 

 hen's body in the nest, or by an incubator, the same sort of 

 development takes place that we studied in the frog. Only 

 in this case there is so much food in the egg that all the 

 steps similar to those which occur in the frog after it 

 hatches take place in the chick within the egg. In other 

 words, the frog has further to go after it hatches than the 

 bird has The bird goes further in its development on the 

 strength of the food which the mother stores in the egg. 



5. The Changes at Hatching. There are some important 

 changes for the chick at the moment of hatching. Up to 

 this time the oxygen, which is necessary for all life, passes 

 through the shell and finds its way into che blood d'rectly 

 by means of some special embryonic membranes. When 

 the chick breaks the shell these membranes dry up and all 

 the oxygen goes into the lungs in the ordinary way. 

 Before hatching all the food has been taken by the blood 

 from the substances in the egg. Almost immediately after 

 hatching the chick begins to practice its instincts for 

 pecking at food, and soon gets to using its digestive tract. 

 Other vital activities follow and the young chick soon 

 performs all the important functions in just the same way 

 the adult does. 



6. The Sexes in Birds. The males and females are 

 always distinct in birds. That is to say, one individual 

 produces sperms and another eggs. Furthermore, the two 

 parents, when adult, are usually quite unlike in appearance, 

 instincts, and behavior. The chicken is a good illustration 

 here, though not better than many other birds. The 

 rooster is larger, more brilliantly colored, more 

 aggressive, more noisy, and has certain structures which 

 the hen either does not have, or not in such degree, as 

 spurs for fighting, comb, wattles, etc. In many birds the 

 male has better developed powers of song. It is thought 

 that many of these qualities in which male and female 

 birds differ are adaptations that enable each the better to 

 meet the work which it must do. For example, the spurs 

 of the cock, his greater size, and his aggressiveness seem 

 to fit in with the fact that in some measure he takes the 

 lead and protects the flock that surrounds him. 



Birds have power to reproduce only by the union of 

 sperms and eggs. 



CHAPTER EIGHTEEN. 

 MAMMALS AND MAN. 



1. The Special Development of Mammals. The 



specialization of mammals, as compared with other 



vertebrates, is chiefly in connection with reproduction and 

 care of young. There is no group in which these functions 

 are more successfully cared for than in mammals. No 

 group has developed higher, finer, more successful instincts 

 and practises. The name mammals comes from the 

 mammary or milk glands by which the young are 

 nourished after birth. . The developing embryo is carried 

 for a considerable period of its early development in an 

 internal organ of the mother. Here it is nourished and 

 protected from the unfavorable conditions outside. It is 

 because of these facts, and the instincts and habits that 

 grow up about them, whereby the bond between the young 

 and the parent is strengthened, that the mammals are so 

 successful and seem to be at the head of the animal 

 kingdom. 



2. Man's Place with the Mammals. As an animal, man 

 is not different from the higher mammals in any very 

 important particulars. He has all the characters of the 

 mammals and some particular ones beside. His 

 reproduction is identical with theirs in all important 

 respects. He cares for his young somewhat better than 

 the others, and educates them longer, gives them a better 

 home, and more sympathy, but the beginnings of 

 everything man does for his children are found in what 

 other mammals do for theirs. 



3. The Gametes in Mammals. As in birds, there is no 

 reproduction in mammals except by the formation of eggs 

 and sperms. The sexes are always separate. The female 

 reproduces by small eggs, much smaller even than those 

 of the frog. Sperm cells are produced by the males in 

 testes just as in frogs and birds. 



The mammals are like the birds in that the eggs are 

 fertilized within the body of the mother. This is not 

 because of a shell about the egg as in the case of birds. 

 It is rather because the embryo when fertilized is going to 

 remain in the body of the female and there develop. It 

 is incubated within the mother instead of in a nest. For 

 this reason the sperms must be inserted into the special 

 female organ, the vagina, by a special act of mating 

 called copulation. From the vagina the sperms ascend 

 and meet the eggs as they come down from the ovaries. 

 The meeting place of sperm and egg is usually the organ 

 in which the young are retained during early development. 

 This organ is the uterus, or womb. It is a special organ 

 formed by a great enlargement of the oviducts or tubes 

 that lead from the ovaries to the outside. 



Fertilization itself differs in no way from that studied in 

 the lower plants and animals. 



4. Early Development of the Embryo. After fertilization 

 the fertilized egg (embryo) divides into two cells, then 

 into four, eight, sixteen, etc., just as in other animals. By 

 steps similar to those studied, the embryos become 

 differentiated into nerve, and muscle, and cartilage and the 

 other tissues. These steps need not be followed again- 

 The point which makes the mammal embryo so peculiar 

 and interesting is this: there is little food in the egg, 

 hence it cannot live as long as the frog's egg or the hen's, 

 on the strength of its own food. So, very soon after 

 fertilization it attaches itself by root-like outgrowths to 

 the tender wall of the uterus. Through these connections 

 it takes up nourishment from the blood of the mother. 

 Thus it grows, literally a parasite on the mother just as 

 the sporophyte of the moss is a parasite on the 

 gametophyte. 



As the heart and blood vessels of the young mammal 

 develop, the latter run out to these connections with the 

 mother just as those of the chick did over the foodstuff 

 the egg. Oxygen as well as food is thus received from the 

 blood of the mother, and the carbon dioxide and other 

 wastes are returned to it. 



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