418 CEPHALOCHORDA, AMPHIOXUS. 



the animal pole. The blastosphere, which is the final result 

 of segmentation, invaginates to form a gastrula. 



Along the mid-dorsal line of the gastrula the ectoderm 

 cells sink in slightly so as to form a groove. This is the 

 medullary groove, which here follows an unusual course of 

 development. Instead of immediately closing to form a 

 canal, the groove sinks inwards, and the lateral ectoderm 

 grows over it before closing takes place. Later, the groove 

 forms the medullary tube, which opens into the gut by the 

 neurenteric canal ; to the exterior by the anterior neuropore. 



The cavity of the gastrula the archenteron becomes 

 the gut of the adult. From it pouches grow out as was 

 described above. 



The notochord arises along the mid-dorsal line of the 

 archenteron ; its forward extension is secondary. 



During the early part of larval life the ectodermal cells 



FIG. 135. Early stages in the development of Amphioxus. 

 (After HATSCHEK. ) 



i. Ovum ; 2. Four cell stage ; 3. Eight cell stage ; 4. External 

 appearance of blastula ; 5, 6. Blastulae in section (note the larger 

 macromeres) ; 7. Beginning of gastrula stage ; 8. Section of com- 

 pleted gastrula. 



including those forming the medullary canal, are ciliated. 

 At this stage the larva is much more active than the 

 adult. 



The later larvae are more sedentary, lying much on the 

 right side, and they are strongly asymmetrical. The mouth 

 is placed at the left side ; the gill slits of one side appear 

 considerably before those of the other ; the primitive seg- 

 ments of one side are not opposite those of the other, and 

 so on. By the process known as the " symmetrisation " of 

 the larva, the apparent symmetry of the adult is produced. 



