PRIMITIVE PORES OF POLYODON SPATHVLA 459 
The primitive pores are simple epithelial invaginations about 
two hundred microns deep and about seventy-five to one hundred 
microns in transverse diameter. Definite measurements are 
given of several pits in connection with fig. 9. The primitive 
pores usually contain some mucous and not infrequently they are 
full of mucous. The mouth or opening is usually smaller than the 
diameter of the pit near its bottom. As a rule each pit has its 
own opening or mouth, but not infrequently two pits, and occa- 
sionally three, have a common opening. In form the pit varies 
from a cylindrical invagination with an almost flat bottom to a 
more or less pronounced flask-shaped depression with a more or 
less concave bottom. None of the pits have what might be called 
a distinct neck. Indeed, as will appear later on, each pit presents 
just two distinct regions — the differentiated bottom and the undif- 
ferentiated region above the bottom. Figs. 8 and 9 show groups 
of primitive pores respectively in transverse and longitudinal 
section. 
As intimated above each primitive pore is more or less com- 
pletely surrounded by branched pigment cells, which are abundant 
in nearly all parts of the body. Fig 8 gives some idea of the dis- 
tribution of pigment cells among the pits near the bottom of the 
pits. Lymph sinuses, blood vessels and nerves are abundant in 
the subcutaneous connective tissue. Figs. 3 and 4 show the 
distribution of the blood vessels in the neighborhood of the primi- 
tive pores of the bill. The vessels are surrounded by a more or 
less dense network of branched pigment cells and consequently 
stand out as black lines. Nerves usually accompany these blood 
vessels but I have never seen a nerve going to the bottom of a 
primitive pore. 
On the ventral surface of the bill there is a double row of groups 
of pores that open into small canals connected with branches of 
the lateral line. These groups are well shown in fig. 2. As a 
rule they are not very evident, but on some bills they are quite 
conspicuous. A comparison of the branchlets of the lateral line, 
shown in fig. 5, with these rather striking structures of the bill, 
shown in fig. 2, will at once suggest that the clusters of pores 
and tubes on the ventral surface of the bill represent close groups 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL., 9, NO. 2. 
