CHAPTER 20 



THIRD PART OF UBER ENTW I C KLUNGSGESCH I CHTE : 



DEVELOPMENT OF THE BIRD EGG AND EMBRYO 



Turning to the structure and formation of the bird's 

 egg, that is, its history prior to hatching, Baer considered 

 the structure of the laid but not yet hatched egg (§ 2) . He 

 successively described the parts of the laid egg, starting 

 with the shell and the underlying two- layered shell membrane. 

 These layers are only separated at the blunt end of the egg, 

 with the air chamber between them. The external layer carries 

 papillae on the external surface, which penetrate into the 

 shell. In the albumen, Baer distinguished three layers under 

 the shell membrane — an external, middle, and internal or third 

 albumen. Concerning what was called the middle membrane of 

 albumen, Baer referred to it as a solitary layer on the 

 surface of the middle albumen, and noted that it is not seen 

 in live eggs and appears only with the effect of water. The 

 same thing apparently also occurs with another formation, the 

 albumen ligament, which Tredern had described in detail (see 

 Chapter 11). Next, Baer briefly referred to the chemical 

 composition of the albumen. In the center of the albumen mass 

 sits the yolk ball, ellipsoid in form; its longitudinal axis 

 corresponds to the longitudinal axis of the shell. 



On the surface of the yolk there is a yolk membrane, 

 composed of a single layer. Of the two layers of the yolk 

 membrane which Wolff had thought existed, the internal one 

 actually represents the embryonic pollicle (blastoderm) . The 

 yolk itself is composed of granules of equal size, irregularly 

 shaped whitish masses and bright light fat droplets. The 

 central cavity in the yolk communicates with the surface by 

 a canal. 1 In front of its external end on the yolk surface 



1. Actually the center of the yolk ball is occupied with 

 what is called the white yolk. 



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