214 THE ORIGIN OF VERTEBRATES 
may contain mucin in the younger developmental stages, and there- 
fore possible that they might at that stage secrete it; they certainly, 
however, show no sign of doing so in their more adult condition, and 
cannot be compared in the very faintest degree to the glandular cells 
of the pharyngeal region. It is also perfectly possible for gland-cells 
belonging to a retrograde organ to become mucus-secreting, and so to 
give rise to the cells of Am- 
phioxus and the Tunicata. 
Tf, then, these cells were 
not retained for digestive 
purposes, what was their 
function? To answer this 
question we must first know 
the function of the corre- 
sponding gland-cells in the 
uterus of the scorpion, which 
undoubtedly secreted into 
the cavity of the uterus and 
took some part in connection 
with the generative act, and 
certainly not with digestion. 
What the function of these 
cells is or in what way they 
act I am unable at present 
to say. I can only suppose 
that the reason why the 
Fig. 93.—A, Portion oF Aa GILL oF AMMO- thyroid sland has persisted 
C@TES WITH ORDINARY RESPIRATORY EPI- iS) 
uptrum; B, Corresponpia Portion or throughout the vertebrate 
THE First on Hyorp Ginn, kingdom, after the genera- 
tive tissues had found a new 
outlet for their products in the body-cavity of the posterior region, 
is because it possessed some important function in addition to that 
connected with the exit of the products of the generative organs; a 
function which was essential to the well-being, or even to the life of 
the animal. We do not know its function in the scorpion, or the 
nature of its secretion in that animal. We know only that physiology 
at the present day has demonstrated clearly that the actual external 
secretion of a gland may be by no means its most important function ; 
in addition, glands possess what is called an internal secretion, viz. a 
\\ 
NW, 
