FORMATION OF THE CHICK 307 



cavity at the upper end of the oviduct, and there, as the naked 

 yolk passes into the oviduct, before albumin has been added, the 

 necessary union takes place. 



It is a common but mistaken belief among the inexperienced 

 that a male bird in the pen is necessary for the production of eggs. 

 This is true so far as the lajdng of fertile eggs is concerned; and, 

 while it is true that the primary object of egg production is repro- 

 duction, j'et impregnation is unnecessary for the production of the 

 egg, and hens will lay just as many eggs when no male bird runs 

 with the flock as they will with one. In fact, it is becoming a 

 rule on the larger egg farms to produce infertile eggs for market, 

 since they are less likely to spoil, and there is no danger of the 

 germ developing and ruining the eggs for eating. Fertilization is 

 not an incentive to egg production among domestic fowls. 



The egg, then, is a productive sac surrounding a female germ 

 cell, which may or may not be fertilized. In the latter case the 

 egg is termed infertile and cannot possibl}^ be hatched, because to 

 produce life there must be the union of the male and female 

 germ cells. 



Formation of the Chick. — The first stage in the development 

 of the embryo takes place before the egg is laid, probably due to 

 the heat from the bird's bodj'. This change is termed " segmen- 

 tation," and results in the multiplication of cells which form a 

 cap over the germ vesicle and a group of cells under it. In the 

 normal egg, development is arrested at this stage, and a certain 

 amount of heat is necessary to renew it after the egg is laid. 



After segmentation and application of the right degree of heat, 

 incubation begins. The germinal disk divides into two layers, 

 between which a third stratum soon forms. The upper layer 

 (called the epiblast) produces ultimately the skin, brain, spinal 

 cord, eye, and internal ear. From the lower layer (hypoblast) is 

 formed the lining of the digestive tract, while from the middle 

 layer (mesoblast) are developed all the other organs, such as bones, 

 nerves, and muscles. This middle layer thickens rapidly, forming 

 two parallel ridges running lengthwise of the germ, with a groove 

 between them which is termed the medullary canal; the walls of 

 this groove gradually extend and finally meet, forming a tube or 

 neural groove, in which the brain and spinal cord develop later. 



The notochord just below the tube can be distinguished from 

 the first day of incubation. It marks out the future bony axis of 

 the body, or the vertebral column. From the notochord are 



