THE FORMATION Ob' THE LAYERS IN AMPHIOXUS. 307 
cells is therefore situated just about where it is in the embryo 
represented in fig. 11. Throughout the sections of this series 
the modifications which the notochordal cells undergo may be 
clearly seen ; we can perceive that the greater part of each cell 
becomes converted into a vacuole, but that the nucleus and 
a ]3iece of the protoplasm immediately surrounding it persists. 
In the old free-swimming larvae a peculiar tube, termed 
Hatschek’s nephridium, can be seen lying above the front end 
of the pharynx and opening into it posteriorly. In 1898 I 
asserted that this tube was the persistent connection between 
the left collar-cavity and the pharynx. This connection lasts 
certainly for a long time, and much longer on the left side 
than on the right, and it is found in the same place as 
Hatschek’s nephridium. It is shown in an embryo of the 
same age as the one we have been considering in fig. 15 as a 
cord of cells with virtual cavity connecting pharynx and left 
collar-cavity {neph.). Owing, however, to the gap which in 
7ny material intervenes between larvm of this age and those 
in which an undoubted Hatschek’s nephridium is present, I 
am not prepared to dogmatically maintain my assertion of 
1898. The oldest stage to which I shall refer in this paper is 
represented by fig. 17 a-h (PI. 5) inclusive, which represents 
eisflit sections taken frotn the same transverse series. The 
specimen which was sectioned was a larva with the mouth and 
first gill-slit open and the club-shaped gland fully formed. In 
the most anterior section (fig. 17 a) the left head-cavity {Ih.) 
is shown to have developed an opening to the exterior. In 
fig. 11 h the external opening of the club-sliaped gland is 
shown lying just ventral to the thickened plate of cells which 
forms the anterior border of the mouth. In fig. 17 c the 
mouth is seen opening on the left side of the larva and the 
cavity of the club-shaped gland can be seen on the right side 
of the pharynx. Beneath the right rudiment of the splauch- 
nocele the right collar-cavity can be seen, and the ecto- 
derm surrounding it is thickened — this is the first visible 
rudiment of the atrial ridge {at.), which eventually meets 
its fellow so as to enclose the atrial cavity. Fig’. 17 d-h 
