NOTES ON EMBRYOS 



323 



and tile two outer rows diverge slightly to come together at 

 the base of the neck, where they continue parallel to the head. 



The inferior tracts consist of two thick rows of sheaths 

 extending along either side of the abdomen, which come to- 

 gether above the furcula. A dense patch fills the apterium 

 over the lower sternum and upper abdomen. 



The head is more or less regularly covered with rows 

 which extend to and surround the nostrils. The eye is en- 

 circled by a widely separated double row of long feathers 

 which connect with the nostril ring bj^ a single row, and the 

 eyelid is fringed with short sheaths. From the nostrils, run- 

 ning along the groove above the eye, is a double row, and 

 midway between the eyes a triple line which extends straight 

 back until just in front of the parietal region, where the outer 

 lines diverge and gradually round together again to enter 

 the spinal tract. The middle line passes into the open space 

 thus formed, where two other rows run parallel with it to 

 the spinal tract. A single line runs behind the auditory aper- 

 ture and joins the ventral tract. The aperture itself is mar- 

 gined with short feathers. (Fig. 110.) 



FIG. 110. HEAD PTERYLOSIS OF AN EMBRYO DUSKY NIGHTHAWK 



