DE VELOPMENT. 



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 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 posteriorly into the 



FIG. 253. Early stages in the development of Amphioxtis. 

 After Hatschek. 



I. Ovum with germinal vesicle; 2. four-cell stage; 3. external 

 appearance of blastulaj 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 ccelomic 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 larvae are more sedentary, lying much on the 



