AMPHIOXUS LANCEOLATUS. 
371 
pore form a double or paired series. A very peculiar fact with 
regard to these fin-rays is that whilst each is connected at its 
base with a strong ridge of connective tissue which forms a 
continuous median plate, springing from the roof of the 
skeletal neural sheath, yet on all its other faces each fin-ray 
is free, lying in a lymph space. The lymph space surrounding 
the fin rays is not a continuous tube but is divided into com- 
partments one to each fin-ray ; and each compartment is lined 
with a pavement of endothelial cells which is extended over 
both the wall of the compartment and the free surface of the 
fin-ray. The liquid in the compartment separating the fin-ray 
from the wall of the compartment is coagulable. The nuclei 
of the cells on the free surfaces of compartment and fin-ray 
may be readily observed in well-stained sections. It appears 
that the compartments filled with lymph are antecedent 
structures to the fin-ray which eventually comes to occupy 
a large part of the space, since in both the anterior and the 
posterior regions of the dorsal fin the fin-rays are relatively 
small and occupy but little of the lvmph-space, whilst at the 
extremities of the series the fin-rays actually disappear entirely, 
leaving only the lymph-holding compartment to represent the 
whole structure. Anteriorly, the fin-ray lymph-space extends 
as far forward in the form of a fine canal as the notochord 
itself, and is divided into five or six compartments devoid of 
solid rays. Posteriorly I have not ascertained its precise 
termination, but there are several compartments overlying the 
last six myotomes which in adult specimens are devoid of fin 
rays. I think that the number of compartments both ante- 
riorly and posteriorly not occupied by fin-rays is larger in 
half-grown than in fully-grown specimens, and that the volume 
and solidity of all the fin-ravs is greater in the more fully- 
grown individuals. Anteriorly, the fin-rays do not commence 
until, in proceeding from before backwards, we have passed 
that region of the nerve-cord which is in relation with the olfac- 
tory pit. The figure given by de Quatrefages (10) of this region, 
being a careful drawing from a living specimen, shows excellently 
the condition of the first few fin-ray spaces and the first rays. I 
