At the beginning of the third day, the chick embryo 

 contains all the existing features inherent to the vertebrate 

 animal in general. By the formation of the urinary sac, the 

 embryo already shows the features of the land vertebrates. 



THE THIRD PERIOD OF CHICK DEVELOPMENT 



Baer divided the third period of chick development into 

 six stages. The first, described in § 9, continues through 

 the sixth and seventh days. At this time the size of the air 

 cavity increases further. The allantois passes the embryo in 

 growth, penetrating between the serosa and the amnion. 



The beginning of the third period corresponds to the 

 first movements of individual embryonic parts. Its form 

 changes somewhat, the curvature in the neck region particularly 

 decreasing. The trunk grows wide as the liver increases and 

 the heart drops, but the volume of the head remains not less 

 than the size of the trunk. The umbilicus now represents not 

 a single opening or ring, but a short canal, which gave Baer 

 the basis to discuss the presence of an umbilical cord in 

 birds. 



The vascular area of the yolk occupies more than half 

 its surface; its vessels (border, ascending and descending 

 veins) begin to disappear. The part of the umbilical vein, 

 which goes to the hollow vein after branching into the liver 

 is comparable, according to Baer, to the ductus venosus Arantii 

 in mammals. 



In the abdominal plates, covering not more than half the 

 walls of the cavity, the rudiments of the ribs assume the form 

 of dark strips. At the upper arches of the vertebrae, bony 

 protuberances form. 



The extremities increase in dimension and distinctly 

 break into four divisions. The elbow and knee joints are 

 directed externally ("as in the majority of amphibia"). With 

 the exception of the terminal divisions, the anterior and 

 posterior extremities are similar, as are the terminal parts, 

 except for differences in the wing, where there are three 

 fingers, and the leg, where there are four or five fingers. 



329 



