DEVELOPMENT OF FIFTH DAY . 541 



develops in the truncus arteriosus. This latter septum is formed be- 

 tween the fourth and fifth pairs of aortic arches and follows a sort of 

 spiral course caudally to where it joins the interventricular septum. It 

 is the position and shape of these septa which cause the blood to course 

 into the different channels as described for the fourth day. 



Two sets of semilunar valves have now formed between the two 

 divisions of the truncus arteriosus and the two ventricles into which 

 they open. The heart continues growing, but it is not until about the 

 twelfth day that the interauricular septum has almost completely closed, 

 leaving only the foramen ovale as a small opening between the two 

 auricles. The foramen ovale develops a little fold of membrane which 

 some days after hatching closes the opening entirely. 



The ventricles now become thickened to a very considerable extent. 

 The auricles likewise thicken, but not to so great an extent as the ven- 

 tricles. The ventricular thickenings on the inside of the heart form as an 

 inward growth of ridges which are called trabeculae carnae. They are 

 really separate muscle-bundles which help to open and close the valves. 



On the sixth and seventh day the distinctly bird-like characteristics 

 appear. Up to this time the beginner cannot tell the difference between 

 a chick embryo and that of practically any other one of the higher ver- 

 tebrates. 



The nasal region now begins to lengthen and the fore-limbs will be 

 seen to develop into wings. 



The allantois has become very large and contains a considerable 

 amount of fluid. 



The omphalomesenteric arteries and veins now pass from the body 

 of the embryo as single vessels. The yolk, though seemingly as large 

 as before, is quite liquid in form. 



The flexion of the body is less marked than before, while the head 

 is not so large in proportion to the remainder of the body as formerly. 



The cerebral hemispheres can be seen quite plainly, as well as the 

 beginnings of the tongue-bud. 



On the next three or four days the little sac-like regions in which 

 the feathers develop make their appearance as protrusions from the sur- 

 face, especially on the dorsal side of the chick, while a chalky patch at 

 the tip of the nose marks the beginning of the horny beak. The yolk 

 has become wrinkled and flabby. 



After the eleventh day, the abdominal walls become firmer and the 

 intestines are enclosed in the peritoneal cavity. The body is now com- 

 plete except for the narrow stalks of the umbilicus and yolk-sac. The 

 amniotic fluid tends to disappear, making the amnion less prominent. 



By the thirteenth day the feathers are well distributed over the 

 entire body, although they do not break through their sacs until about 

 the nineteenth day, by which time they are approximately an inch in 

 'ength. 



