DEVELOPMENT. 



421 



micromeres) above or towards the animal pole. The blasto- 

 sphere, 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 



Fig. 182. — Early stages in the development of Ampkioxus. 

 — After Hatschek. 



1. 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. 



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

 neurenteric canal, and to the exterior by the anterior neuro- 

 pore (Fig. 183). 



The cavity of the gastrula — the archenteron — becomes 

 the gut of the adult, and gives rise to the coelomic pouches 

 (seep. 417). 



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. 



