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 cesophagus, 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 
