84 Observations upon the Habits, Structure and | February, 
sides in the direction of the axis which passes through the gas- 
trula mouth, or blastopore, changes into a slender, compressed, 
double-walled, planula-like body, ciliated within and without, and 
with the two walls, or exoderm and endoderm layers, which sur- 
round the rather large central cavity of invagination, lying close 
together one within the other, but separated by a narrow space 
which represents the remains of the original segmentation cavity 
of the morula. By the time the embryo is well established in 
its planula form, the blastopore, which marks the posterior 
extremity of the body, closes up entirely, or at least it is highly 
probable that it does, and the young Amphioxus escapes from 
its shell and becomes a free swimming inhabitant of the water. 
The cells of the exoderm and endoderm, along one edge of the 
embryo, now become longer and larger, entirely obliterating in 
their growth the segmentation cavity in this part of the body, 
-and form between them, throughout the length of the animal, a 
strip of mesoderm in the center of which arises the notochord 
and from the sides of which originate the muscle plates of the 
body muscles. As the mesoderm thickens, two longitudinal 
ridges grow up from the exoderm and, arching over, unite upon ` 
the median line into a dorsal tube which runs parallel with and 
close above the notochord. Within this tube the central portion 
of the nervous system is formed. This arises as a second tube 
lining the walls of the first, and originates from the differentia- 
tion of the exoderm cells of the latter. During its formation the 
ends of the dorsal canal have been gradually closing up, one of 
them, the posterior, completely, and the other, the anterior, all 
except a narrow outlet which persists until a somewhat later 
stage of development. 
Meanwhile the whole body lengthens; the exoderm of the | 
ends stretches away from the endoderm into thin blade-like 
points; the central cavity, limited by the transformation of the _ 
dorsum to the lower half of the body, becomes long and tube- — 
like posteriorly, and quite broad throughout its anterior third; a 
welt or pear-shaped body, with the narrow end pointing down- 7 
wards, forms across the anterior end of this broad pharyngeal = 
portion of the central cavity, and the external cilia, which here- _ 
tofore have been the sole motive power, supplanted | in their func- 
tions by the muscular fibres of the mesoderm, disappear, except a 
= e a OS Taraa P -like mt of SHEE situated aa the 
