I30 



CEPHALOCHORDATA 



Embkyology and Life-Histoey. 



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 



c 



A B 



Fig. 82. — Stages in the segmentation of Amphioxus. A represents the eight-celled stage ; 

 B, the si.xteen-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 



