DEVELOPMENT. 



449 



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 

 gut by a "neurenteric canal," and to the exterior by the 

 anterior neuropore (Fig. 222). 



The cavity of the gastrula the archenteron becomes 



FIG. 221. Early stages in the development of Awphioxus. 

 After Hatschek. 



i. 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. 



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



(see p. 445). 



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 



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- 



29 



