KiSTLER, Primitive Pores of Polyodon. 295 



unaided eye by reflected light. The tissues were then washed for 

 one hour in distilled water, run rapidly through the different grades 

 of alcohol and embedded in paraffin. Sections from 3// to 5 /< were 

 cut and mounted in balsam. 



On casual examination, the head, bill and gill-flaps of Polyodon 

 appear to be profusely covered with irregular dark spots (Fig. i), 

 which are much lighter on the gill-flaps than on the head and bill. 

 When these spots are examined more closely, they are seen to con- 

 tain small pin-point holes which are the openings of primitive 

 pores. The pores are exceedingly numerous, there being as many 

 as fifty in most of the spots. No spots, however, are found in a 

 strip I cm. to 2 cm. wide which extends along the mid-dorsal sur- 

 face of the head and bill, and also in a similar strip of the mid- 

 ventral surface of the bill, as show^n in Fig. i. 



The pores are arranged in small groups of from three to twenty 

 each. From the counting of some fifty of these groups, they were 

 found to contain an average of about eleven pores. From two to 

 ten of these smaller groups are collected into larger groups which 

 measure from 2 mm. to 12 mm. in diameter and correspond to the 

 above-mentioned spots. These larger groups are usually circular 

 or ovoid, except on the gill-flaps where they become more or less 

 linear. Close around the eye and nose, where the pores are very 

 numerous, the larger groups are not well defined, but the smaller 

 ones are as distinct as elsewhere. The smaller groups of one fish 

 were counted and found to be distributed as follows: On the 

 dorsal surface of the bill, 1845; o" ^^^ ventral surface of the bill, 

 1648 ; on the head, 760; on the gill-flaps, 926. Taking 11 as the 

 average number of pores in each of the smaller groups, the approx- 

 imate total number is 57,365. 



The pores usually open singly, but in some instances, two or 

 'even three pores have a common opening. The form of the open- 

 ing when single is circular or oval with a diameter of from 0.3 mm. 

 to 0.5 mm. Sections show that the depth of the pores is from three 

 to five times the thickness of the epithelium, instead of being no 

 deeper than the epithelium asshownby Collinge (i, PI. 39, Fig. 5). 

 Serial sections show that the general form of the pores is cylin- 

 drical. In some the lumen diminishes toward the base of the pore, 

 becoming conical, while in others it increases toward the base of 

 the pore becoming flask shaped. Occasionally a pore has a 

 lateral sacculation. 



