116 THE STRUCTURE OF THE FOWL 



cavity. By extension of the process of seg- 

 mentation in a peripheral direction, a disc of 

 cells, the blastoderm, is produced before the 

 egg is laid. In some manner, not as yet 

 perfectly understood, the cells of the blastoderm 

 are differentiated into two strata — a superficial 

 ectoderm and a deeper entoderm. When the 

 egg is laid and the temperature falls in conse- 

 quence, development of the blastoderm is 

 arrested until such time as the temperature is 

 again raised by incubation. 



The unincubated blastoderm is a whitish 

 disc, some 4 mm. in diameter, which presents 

 a central and more transparent area, the 

 area pellucida, over the region of the sub- 

 germinal cavity, and a peripheral and less 

 transparent zone distinguished as the area 

 opaca. The embryo itself develops within 

 the area pellucida ; while from the rest of 

 the blastoderm spring those extra-embryonic 

 membranes whose function is concerned with 

 respiration, nutrition, and protection. 



The first four days of incubation are marked 

 by the rapid extension of the blastoderm, 

 which, by the fourth day, has spread beyond 

 the confines of the original germinal disc and 

 covers a great part of the surface of the yolk 



