FORMATION OF THE CHICK 309 



day's growth ; these are essential for the nourishment of the embryo, 

 hence they are early active, delivering a constant blood stream. 



The progress during the third day (Fig. 149) is in the nature of 

 continued development of structures already formed rather than 

 in the formation of new ones. Up to this time the embryo has 

 been lying face downward, but it now turns on its left side with 

 the head bent downward. The vitelline circulation is completed, 

 the heart is subdivided and further developed, and additional 

 arteries and veins are formed. Some of the special senses are now 

 formed or developed ; among the most noticeable are the formation 

 of the lens of the eye and the development of the nasal pits. The 

 cerebral hemispheres are more clearly defined, the hindbrain sepa- 

 rates into the cerebellum and medulla oblongata, and many of the 

 larger nerves branch out from the nerve centres. The alimentary 

 canal increases in size and its parts are more clearly differentiated; 

 the oesophagus, stomach, and duodenum becoming well defined. 

 The lungs now make their appearance as an outgrowth from the 

 alimentary tract. By the third day the allantois can be outlined; 

 it is a temporary membranous structure closely associated with 

 the alimentary tract, and from it is soon developed a complete 

 system of blood vessels which carry blood to and fro. On the 

 ninth day it is well developed, and can be seen spreading over 

 the back of the embryo, close to the shell. It is the medium through 

 which respiration is effected in the embryo, and at hatching time 

 it shrivels up and is cast off. 



The fourth day is characterized by a rapid increase in the size of 

 the embryo, combined with a noticeable diminution in the amount 

 of albumin, and the embryo lies very close to the shell. The vascu- 

 lar area at this time is about one inch in diameter and carries an 

 increasing amount of blood. The body proper develops rapidly, not 

 only in size but in the formation of the limbs, the legs being short 

 and thick, while the wings are long and slender. In the female 

 rudimentary sexual organs make their appearance on this day. 



From the fifth day the growing embryo reveals development 

 rather than the formation of structures, since these are now estab- 

 lished in greater or less degree. On the fifth day, certain cartilaginous 

 processes can be detected, which subsequently develop into bones. 



The sixth and seventh days mark that special development in 

 the embryo of the fowl which is to distinguish it from that of a 

 mammal or reptile, for in the first four or five days the develop- 

 ment of all vertebrates is very similar. After this time the more 



