CHAPTER 17 



FIRST PART OF UBER ENTW I CKLUNGSGESCH I CHTE 



DEVELOPMENT OF THE CHICKEN IN THE EGG 



In discussing the history of chick development, Baer 

 began with the first period of development of the incubated 

 egg. To this description he prefaced a short introduction. 

 Following Pander, Baer indicated that the temperature 

 necessary for chick embryo development ranges from approxi- 

 mately 28° to 32° C. The temperature conditions distinctly 

 affect the speed of development. Apparently contradictions 

 in the data of different authors about the periods of 

 development result from that difference; for even Wolff, 

 who in Baer's words was an extremely accurate observer, had 

 such contradictions. 



Baer differentiated two types of deviations from typical 

 development: irregularity of the general course of develop- 

 ment in different eggs, and irregularity in the realization 

 of different features in the same egg. Greater or less speed 

 of development may depend upon the time of the year as well 

 as the temperature of incubation, and especially upon how 

 long the egg is kept before incubation. 



Depending on how long the egg is kept, slowing of 

 development may be fairly significant, especially during the 

 first five days of incubation. Taking into consideration 

 the existing variations, Baer divided development into three 

 periods. The first period lasts approximately two days and 

 ends with the complete formation of blood circulation. The 

 second period has a duration of three days and involves 

 circulation of the blood through the vessels of the yolk 

 sac. The third period continues through the appearance of 

 blood circulation in the lungs, that is, until hatching. 



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