466 ALLIS. [Vor 28, 
into some one of the dermal bones. In its course it unites with 
similar canals from other pores, and increasing continually in 
size, finally becomes a trunk canal, which opens into one of the 
main lateral canals of the head. This trunk canal and its 
branches form what has been called a dendritic system, of which 
the surface pores are the external openings. 
The arrangement of these pores varies greatly in different parts 
of the head. They are found either single or in pairs, in more 
or less irregular lines, or in irregular groups, the number and 
arrangement of them differing somewhat in every specimen, and 
even on opposite sides of the head in the same specimen. As 
many as thirty-seven hundred were counted on the head of a 
single large specimen, and the number apparently increases 
indefinitely with the age of the fish. This great multiplication 
is the result of a repeated dichtomous division of the pores 
formed in younger stages. The impulse leading to this division 
acts with much greater intensity in certain parts of the head 
than in others, but is felt in all parts; for, with the exception 
of the large pore at the hind edge of the supraclavicula (Fig. 
39, Pl. XXXIX., 2722/19), which marks the limit of the cranial 
system, there is not a pore on the head of the young fish that 
is not double in later stages. 
In both fresh and alcoholic specimens, the pores have a well- 
marked whitish border, due mainly to the absence of inter-epi- 
thelial pigment cells; and when fully formed they are approxi- 
mately round. When about to divide, the sides of the oblong 
and continually lengthening pore grow rapidly upward and toward 
each other, and the pore becomes hour-glass or dumb-bell-shaped. 
The projecting sides or lips finally meet and coalesce, and the 
pore is divided into two pear-shaped portions which lie in a 
small unpigmented oval space. In this process the canal lead- 
ing to the pore is also divided into two portions, for an arch 
is formed across the end of it, and it becomes Y-shaped form 
as shown in the accompanying diagrammatic figure (Cut 1, d, 
é, f). The two newly formed pores are at first connected by 
a whitish cicatrice (Fig. 17, Pl. XXXV.); but the impulse which 
led to their formation continuing to act, they travel apart, the 
cicatrice disappears, and they become distinct and perfect pores. 
One or both of them may again divide in the same manner, 
and nearly in the same direction as at first, thus giving rise 
