DEVELOPMENT. _ 469 
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 coursé 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 posteriorly into the 
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Fic. 253.—Early stages in the development of Amfhioxus. 
—After Hatschek. - 
x. Ovum with germinal vesicle; 2 four-cell stage; 3. external 
appearance of blastula; 4. blastula in section; 5. beginning of 
gastrula stage ; 6. section of completed gastrula. 
gut by a “neurenteric canal,” and to the exterior by the 
anterior neuropore (Fig. 254). 
The cavity of the gastrula—the archenteron—becomes 
the gut of the adult, and gives rise to the coelomic pouches. 
The notochord arises along the mid-dorsal line of the 
archenteron ; its forward extension is secondary. 
‘During the early part of larval life the ectoderm cells, 
including those forming the medullary canal, are ciliated. 
At this stage the larva is much more active than the adult. 
The later larvee are more sedentary, lying much on the 
