294 CHORDATA. 
presence of the atrium, Ascidéa can be directly compared 
with the other class of the Azrtozoa. 
The conclusions drawn from these characters have, how- 
ever, an ample corroboration in the development. 
The eggs are laid into the atrium, in which they are 
fertilised and pass their early stages. Later, the larva is 
free-swimming and pelagic. i 
The segmentation is total and nearly equal, producing a 
blastula which is invaginated to form a gastruda. The 
gastrula elongates, and the blastopore comes to lie in a 
postero-dorsal position in relation to the adult axes. From 
Fig. 206.—TRANSVERSE SECTION THROUGH EMBRYO OF AN 
ASCIDIAN. 
(After DELAGE.) 
Neural Groove. 
Norochord: Mesoblastic 
Sac. 
Epiblast. 
Archenteron. 
the blastopore forwards to the anterior end of the gastrula 
the median dorsal line of cells.becomes the dorsal nervous 
system, which is at first dermic, but it is transformed into a 
long dorsal nerve-tube by invagination -proceeding from 
behind forwards. The front end of the tube, called the 
neuropore, is open, and the posterior end, leading through the 
blastopore into the archenteron, is known as the neurenteric 
canal. Meanwhile the hypoblast has been developing. 
The hypoblastic cells lying in the mid-dorsal line immedi- 
ately below the neural tube become pinched off from the 
rest to form a long rod-like body, the zotochord. Laterally 
to this organ are paired pouchings of the hypoblast which 
give rise to the mesob/ast or third embryonic layer. Their 
lumen is soon lost, and the mesoblast comes to lie as a pair 
of lateral masses of cells between epiblast and hypoblast. 
We now have the ¢ypical chordate larva or Chordula, con- 
sisting of an elongated body, with a long dorsal nerve-tube, 
opening anteriorly to the exterior, posteriorly into the arch- 
enteron, a median dorsal notochord separated from the 
