THE PIGEON 191 



(2) Boil an egg about four minues, or until it is hard. 

 Note the outside covering first with the unaided eye, then 

 with the magnifying glass. What is the general shape of 

 the egg? What is the nature of the surface? Break the 

 eggshell and pick it off without injuring the membrane be- 

 neath. Study this membrane as you did the outer shell. 

 Does it have the same general appearance? Does it have 

 the same texture? Do you think this membrane has grown 

 to its present size? Has the shell grown, or was it merely 

 deposited? Cut the egg across at right angles to its longer 

 axis. What do you find in its composition? Draw (X)^)- 



(3) After the egg is deposited it remains inert until 

 subjected constantly for a considerable time to a temperature 

 equal to that of the body of the bird. How is this secured? 

 When this condition is reached, the embryo which has al- 

 ready begun to grow, enlarges, using in the meantime the 

 yolk and the white for food, until finally it bursts the shell 

 and comes forth in all respects like the adult except in size. 

 In its immature condition it is fed by food prepared by its 

 parents until it is able to select its own food, 



(4) In the development of birds the egg during the 

 period of incubation passes through the fundamental process 

 of segmentation, as. do the eggs of all other animals. The 

 first layer of cells developed on the outside of the yolk 

 is called the hlastula, or blastoderm, which, after folding 

 in at one end leads to the development of the three primitive 

 germ layers already referred to, the ectoderm, the mesoderm 

 and the endod.erm (called by some authors, respectively, 

 epiblast, m.esohlast and hypoblast). 



From the ectoderm arise the cuticle and its modifications, 

 the entire nervous system, including the brain, spinal cord 



