FIRST PERIOD OF CHICK DEVELOPMENT 



Baer's description of the first period began with the 

 events of the first day of incubation (§ 1) (Figure 27) . He 

 knew that development of the chick embryo begins even before 

 incubation. Therefore he described the developmental process 

 as a continuation of the separation, already begun, of the 

 embryo from the yolk and the primordial membrane. (92) The 

 embryo at that time becomes more compact and thin, and its 

 center is raised in the form of an overgrowth. At about the 

 twelfth hour of incubation one can detect the division of 

 the primordial membrane into two layers: the surface layer, 

 which is thin and dense, and a deeper layer, thicker and 

 looser. Baer, in agreement with Pander, named the first 

 layer "the serous layer" and the second "the mucous layer." 

 Simultaneous with that separation, separation is also observed 

 along the surface, so that the center becomes lighter than 

 the borders. The light part (the area pellucida) changes 

 form from rounded to oval, and then into a pear shape. 



With the continuing separation of the rudiment from the 

 underlying yolk, as a result of which the sub-embryonic cavity 

 forms, 1 further separation in the rudiment membrane occurs: 

 the dark border, surrounding the transparent embryonic area, 

 is divided into concentric rings. At the same time, between 

 the serous and mucous layers, a third "vascular layer" develops; 

 therefore, Baer gave it the name vascular area (area vasculosa) . 

 The external ring adjoining the yolk he designed "the yolk 

 area" (now called the "area opaca") . 



After the fourteenth or fifteenth hour of incubation, the 

 first sign of embryonic organization appears, which leads to 

 the appearance of the dark zone. Baer gave this formation the 



1. It is possible not to stop at Baer's wrong idea about 

 nutrition of the embryo by the fluid which, in his 

 opinion, collects in the cavity of the white yolk, 

 but which actually does not exist. 



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