THE BIRDS . 95 



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 

 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 



