56 MORGAN. [Vor Exe 
the third body-cavities, but not above, owing to the develop- 
ment of the cross-rods uniting the bars. 
A few other observations may be recorded for this stage. 
A small fifth pair of gill-slits is forming as a pouch from the 
upper corners of the oesophagus behind the fourth pair. of 
slits. Its tongue-bar is forming by inturning of the dorsal 
wall. The cells of the stomach-intestine immediately behind 
the oesophagus are arranged in pouches formed by short fold- 
ings of the wall of the digestive tract. The cells in the folds 
are becoming more columnar, have conspicuous nuclei, and 
are filled with granular protoplasm. These are the cells that 
give the brownish color to the anterior region of the body, 
and are commonly spoken of as liver-cells. 
The notochord is large, and fills up much of the interior 
of the narrow neck. In this region the mesenchyme cells 
that formed so conspicuous a feature in the earlier stages are 
no longer noticeable, and the cavity in which they were formed 
along each side of the neck is obliterated. They are caught 
between the walls, and some at least break down and disap- 
pear. 
Around the posterior edge of the collar a collection of punctated 
substance marks the course of a nerve-tract that arises from the 
posterior end of the nerve cord, or rather 1s continuous with tt. 
The nerve cord is longer and also broader than in the last 
figure. 
A blood-vessel runs around the surface of the anterior por- 
tion of the oesophagus. Dorsally it seems to be continuous 
with the mid-dorsal vessel, but below I could not trace its 
connection with the ventral vessel. 
At the tip of the proboscis a few pigmented nuclei mark 
the former position of the larval eyes. 
The reproductive organs appear at this stage, but they will 
be described later. 
A few words may be added with respect to young worms 
standing in point of age between the last stage and the oldest 
larvae obtained. Six gill-slits have opened to the exterior. 
The main features of the preceding stage are to be found 
