Jones — On Common and Roseate Terns. 



41. 



Fisrure 3. 



a perfect feather. The down is seen to form a fringe along 

 the tip of the flattened feather (Fig. 3), each down barb being- 

 attached to one or more of the barbs of the new feather. This 

 down fringe wears off rapidly on all exposed parts, but is 

 sometimes found in protected places on young birds some days 

 after they begin to fly about. The young bird can fly feebly 

 by the eighth day after hatching. 



The embryo, on the 18th 

 day of incubation, lies doubled 

 up in the egg, with the head 

 turned under the body rest- 

 ing on the abdomen, just in 

 front of the elongated yolk 

 sac. The bill is thrust betweeii 

 the right knee and elbow, in- 

 cliniing slightly toward the 

 body, tlie left foot touching 

 the neck, just back of the ear, 



the right foot resting on the 

 Juvenile definitive featliers of r , , , , , , • .i „ 

 „ , . , . , , , , forehead, almost touchmg the 



S. luntndo with down barbs at- 

 tached to their tips, "&'"'t ^Y^- 



By the 18th day of incubation the shell is cracked in one or 

 more places near "the large end. The cracks are radially ar- 

 ranged in areas, and are clearly produced by pressure upon 

 the shell from within. Examination reveals the fact that be- 

 neath each cracked area lies some protruding part of the em- 

 bryo — the knee, bill, or shoulder — which is in contact with the 

 shell when the embryo has a muscular contortion. A faint 

 peeping can be heard at this time. 



On the 19th, in a few instances on the 20th day of incubation, 

 the shell was found broken through by the tip of the bill for a 

 space the diameter of the tip of the bill. This break was usu- 

 ally near the center of the first seen cracked area. I did not 

 find any instance in which this air hole was materially en- 

 larged, nor any instance in which any other holes were made 

 in the shell. On the 21st day of incubation the nuiscular con- 

 tortions of the chick resulted in bursting- off the entire large 



