Metazoa A mphioxus. 



261 



tube is still open in front, however, and is in communication 

 with' the gastrular cavity, or archenteron, by the blastopore. 

 The narrowTopen limb will become the spinal cord of the 

 adult, and will become separated from what is destined to 

 be the enteron, or alimentary canal below. 



Meantime from the walls of the archenteron there have 



FIG. 127. DEVELOPMENT OF Amphioxus lanccolatiis. (After B. Hatscheck.) 



A, blastosphere ; B, gastrula ; c, transverse section of the embryo ; D, 

 longitudinal section of the embryo ; a-, ectoderm ; />, endoderm ; c, 

 enteric cavity ; ef, neural canal ; e, mesoblastic somites ; f, neurenteric 

 canal ; g, anterior opening of the neural canal ; h t notochord. 



been forming a series of saccular outgrowths, which ere long 

 become disconnected from the archenteron, and lie along 

 its sides as cubical bodies, known as the mesoblastic 

 somites. Each block becomes subdivided into a dorsal 

 and a ventral portion. The dorsal part becomes transformed 

 into the muscle plates, while the ventral portion splits into 

 two layers : the inner, or splanchnic layer, forming the 



