1880. | Development of Amphioxus lanceolatus. 87 
chioporic depression would seem to indicate some such develop- 
ment, and in this case the lateral abdominal folds, the metapleura 
of the adult, would represent the external edges of the first pair 
of reduplications. This point will, however, be rather difficult to 
ascertain, and I am obliged to say that in none of my young 
specimens did I see any direct evidence of any such secondary 
growths, the development appearing to be as first described, so 
that I shall continue to consider the cavities as formed only of 
two folds. In whichever manner, however, they are formed, the 
one slender and of little account, being barely discernible, the 
other large and of great importance in the animal’s economy, 
they are both lined, from the nature of their formation, with a 
continuous layer of exoderm, that upon the external walls of the 
cavities being derived by cell multiplication directly from that 
which covers the inner walls. At this périod the right fold extends 
forward as far as the edge of the cartilaginous welt, where it 
merges into the exoderm, the left, as yet, only to and uniting 
with the edge of the mouth ridge, both being perfectly free 
from the sides or lower ‘edge of the pharynx, and by the time 
they haye united along their ventral edges to a short distance in 
front of the branchiopore, two or three additional pharyngeal 
clefts have formed along the median line of the pharynx, and the 
animal now appears much as shown in Fig. 7, Pl. 11, which repre- 
sents the youngest of the specimens which came into my posses- 
sion: Here can be seen the long notochord with the slender, 
tubular, spinal axis above it; the ciliated pit just in front of the 
pear-shaped welt; the side mouth with its teeth or tentacles; the 
cilia lining the alimentary tract; the long cesophagus, the dilata- 
tion for the stomach and the asymetrical anus forced to one side by 
the outgrowth of the exoderm of the tail and the median ridge — a 
of the ventral folds; the depression indicating the position of the oe 
branchiopore ; the three pharyngeal clefts piercing the pharynx 
beneath the mouth aperture, and the long tubular heart, formed 
at about this period, between the endoderm and exoderm of the 
a 
ventral edge of the alimentary tract. The shape of the animal is 
quite characteristic, very much compressed from side to side and 
Pointed at either extremity, but the posterior end is not often. 
<nobbed, as shown in the plate, having generally the shape shown: 
in Fig. 5, Pl. 1. The full number of clefts now soon appear along 
i of the side 
; . og lower edge of the pharynx between the open edges of th 
