526 



CEPHALOCHORDA 



(Fig. 301). The first cleavage is vertical, and divides the 

 ovum into two equal parts ; the second is also vertical, 

 along a meridional plane at right angles to the first, and 

 the result is four equal cells. The third cleavage is equa- 

 torial, and gives rise to four larger cells (or macromeres) 

 below or towards the vegetative pole, and to four smaller 

 cells (or micromeres) above or towards the animal pole. 

 The blastosphere, which is the final result of segmentation, 

 invaginates to form a gastrula. 



Fig. 301. — Early stages in the development of Amphioxus. 



— 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. 



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 

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

 anterior neuropore (Fig. 298). 



