Because of its expansion, the serous layer of the head 

 cap completely covers the heart at the end of the second day, 

 At the same time as the increase of the head cap, a caudal 

 cap appears as a fold of the blastoderm; hence, in the 

 posterior part of the embryo the mucous layer forms a pit 

 which is the rudiment of the posterior part of the digestive 

 canal. 



The dropping of the abdominal plates leads to the 

 subsequent separation of the embryo from the blastoderm. By 

 the end of the second day, the place of the future mouth 

 opening is seen; the head end of the embryo still bends 

 downwards. 



The general cnaracteristic of the first period of 

 development, Baer stated in § 3: "(The history of the first 

 period shows that) the embryo is a part of the primordium 

 which arises to a higher independence; that as this 

 independence becomes expressed, the vertebrate type and 

 development above and below that family stem emerges; and 

 that there appears in the animal part the articulation which 

 is characteristic of the . . . type" (I 3, p. 38). 



SECOND PERIOD OF CHICK DEVELOPMENT 



Baer considered the second period to begin with the 

 appearance of blood circulation through the yolk vessels and 

 to end with the development of the urinary sac, as the organ 

 of respiration. This period extends from the third to the 

 fifth day of incubation, and during this period the separation 

 of the embryo and the formation of the temporary embryonic 

 membranes also occurs (§ 4). 



The events of the third day of incubation are stated in 

 §5. In this period only the separation of the embryo occurs, 

 leading to the development of the chest and the lower part of 

 the body, and at the same time the formation of the intestine 

 and mesentery. The chest as well as the abdominal cavities 

 form by the changing situation of the abdominal plates; the 

 external borders of the abdominal plates become inclined 

 increasingly downwards. At the same time the abdominal 



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