12 EARLY EMBRYOLOGY OF THE CHICK 



can be differentiated, white yolk, and yellow yolk. Aside from 

 the difference in color visible to the unaided eye, microscopical 

 examination will show that there are differences in the granules 

 and globules of the two types of yolk, those in the white yolk 

 being in general smaller and less uniform in appearance. The 

 principal accumulation of white yolk lies in a central flask- 

 shaped area, the latebra, which extends toward the blastoderm 

 and flares out under it into a mass known as the nucleus of 

 Pander. In addition to the latebra and the nucleus of Pander 

 there are thin concentric layers of white yolk between which lie 

 much thicker layers of yellow yolk. The concentric layers of 

 white and yellow yolk are said to indicate the daily accumula- 

 tion of deutoplasm during the final stages in the formation of 

 the egg. The outermost yolk immediately under the vitelline 

 membrane is always of the white variety. 



The albumen, except for the chalazae, is nearly homogeneous 

 in appearance, but near the yolk it is somewhat more dense 

 than it is peripherally. The chalazae serve to suspend the yolk 

 in the albumen. 



The two layers of shell membrane lie in contact everywhere 

 except at the large end of the egg where the inner and outer 

 membranes are separated to form an air chamber. This 

 space is stated (Kaupp) to appear only after the egg has 

 been laid and cooled from the body temperature of the hen 

 (about io6°F.) to the ordinary temperatures. In eggs which 

 have been kept for any length of time the air space increases 

 in size due to evaporation of part of the water content of 

 the egg. This fact is taken advantage of in the familiar method 

 of testing the freshness of eggs by ''floating them." 



The egg shell is composed largely of calcareous salts. These 

 salts are derived from the food of the mother and if lime con- 

 taining substances are not furnished in her diet the shell is 

 defectively formed or even altogether wanting. The shell is 

 porous allowing the embryo to carry on exchange of gases with 

 the outside air by means of • specialized vascular membranes 

 arising in connection with the embryo but lying outside it, 

 directly beneath the shell. 



Incubation. — When an egg has been laid, development ceases 

 unless the temperature of the egg is kept nearly up to the body 

 temperature of the mother. Cooling of the egg does not, how- 



