306 CHORDATA. 
Development.—The eggs are shed through the atriopore to the 
exterior, where they are fertilised. Segmentation is total and equal and 
results in a blastula which in its turn is converted into a gastrula by 
archiblastic invagination. The gastrula then elongates, the blastopore 
taking up a postero-dorsal position. 
The epiblast then invaginates along the mid-dorsal line to form a 
nerve-tube and the hypoblast gives rise to a median dorsal notochord 
and paired lateral mesoblastic sacs. In this manner is produced a 
chordula larva practically similar to that of Ascidia. The main dis- 
tinction lies in the origin of the mesoblast. Instead of a single pair of 
somites which rapidly become a pair of solid mesoblastic masses, event- 
ually breaking up into scattered cells, there are in Amphioxus a great 
number of somites, each of which has a definite ccelomic cavity. It is 
Fig. 220.—TRANSVERSE SECTIONS THROUGH YOUNG AMPHIOXUS, 
SHOWING DEVELOPING ATRIUM. 
(After LANKESTER and WILLEY, Bovenrt, and others.) 
Mesoblastic 
Sheath of 
Notochord, 
Sclerotome. 
Myotome 
Sclerotome. 
Gonotome. 
Perivisceral’ - 
Ccelom. ug 
6 
De 
g8 
[fS) 
Metapleural = 
Cavity. 3) 
a Atrium. 
Atrium. 
Ventral Muscle. 
said that one pair of pre-oral somites arise from the anterior end of the 
archenteron, a second pair behind these, called the col/ar-sacs, and a 
third pair at the posterior end laterally to the blastopore. The pre- 
oral pair form the head-cavity (right) of the larva and the pre-oral pit 
(left). Each of the collar-somites divides into a dorsal portion, which 
forms the first myomere muscle, and a ventral part forming the meta- 
pleural cavity. Lastly, the posterior somites divide up to form a great 
number of mesoblastic somites: so far as is known, they alone are 
found in Asczdia. 
The three pairs evidently correspond to the three archimeric seg- 
ments of Balanoglossus and the other Archicelomata, and the metameric 
segmentation of the Chordata is clearly produced by secondary segmen- 
tation of the posterior segment or opisthomere. 
