The urinary sac, in the form of a closed vesicle, covers 

 a great part of the yolk sac. One surface of the allantois 

 lies adjacent to the amnion and -yolk sac; the other surface, 

 richer in blood, is adjacent to the serous and shell membrane. 



At the ninth and tenth day, the feather appears on the 

 skin cover,- at first on the middle of the back and thighs; 

 the rudiments of the rudder feathers are especially marked. 

 The differences between the anterior and the posterior 

 extremities become more distinct. The elbow now is directed 

 backwards, and the knee anteriorly; after the eighth day, the 

 fingers are differentiated; on the tenth day, the terminal 

 divisions of the extremities completely acquire the features 

 of the wings and legs . 



The abdominal plates in the anterior part of the trunk 

 become closed. At the place of the closure or junction, the 

 chest becomes established. Baer followed the trunk nerves 

 throughout their extension during this period. In connection 

 with the study of the development of nerves, he raised the 

 question, do they grow out of the spinal cord or grow into it? 

 The centrifugal direction of development of the head nerves 

 ("nerves of the sensory organs"), Baer presumed, does not 

 suggest that the same method of development occurs in the 

 spinal cord nerves. He suggested that the development of the 

 nerves does not take place in either the central nervous system 

 or in the muscles. He wrongly concluded that the nerve develops 

 along all its extension at once by its separation from the 

 producing tissue. 



The muscle fibers become noticeable after formation of 

 the cartilage. At the beginning, they appear in the region of 

 the thighs and shoulders, and then in the forearm and shanks. 

 Ossification begins in the extremities, first in the large 

 tibial bone, then somewhat later in the femur and fingers of 

 the feet . 



The internal structures change in this stage more than in 

 others as a result of the backward displacement of the heart, 

 liver and stomach. 



The glandular stomach is still not clearly distinct from 

 the gizzard, which in general is characteristic of wild birds. 

 The distinct separation of these parts, characteristic of 



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