130 CEPHALOCHORDATA CHAP. 
EMBRYOLOGY AND Lire-HISTorY. 
Development takes place in the sea-water where the egg is 
fertilised—apparently always about sunset, the embryonic stages 
being passed through during the night, and the larva hatched in 
the early morning. 
The egg is small (0°105 mm. in diameter when shed) and 
contains very little food-yolk. Segmentation is complete (Fig. 
82, A), is nearly regular, and results in the formation of a hollow 
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Fia. 82.—Stages in the segmentation of Amphioxus. A represents the eight-celled stage ; 
B, the sixteen-celled ; D, vertical section of C; F, vertical section of the blasto- 
sphere or blastula stage (E). (From Korschelt and Heider, after Hatschek.) 
blastosphere (Fig. 82, E, F), the wall of which is one cell thick. 
The lower cells (Fig. 82, B, C, D) are slightly larger than the 
upper. Invagination of the lower cells then takes place (Fig. 83, 
A), resulting in the suppression of the blastocoele or segmentation 
cavity and the formation of an archenteron, at first shallow and 
opening widely to the exterior (Fig. 83, B), and then deeper and 
with the opening narrowed to a small posterior blastopore (Fig. 
83, C). This “ gastrula” stage differs from the blastosphere in 
having a mouth or blastopore, and in being two cell-layers thick 
—epiblast (ectoderm) on the outside and hypoblast (endoderm) 
within. It soon shows the future aspects of the body by its 
