CEPHALOCHOKDA 



145 



the blastoporal rim is more strongly manifested dorsally at 

 the region which is called the dorsal lip, and a flattening of 

 the embryo takes place just in front of this part of the blast o- 

 pore. This is the prelude to the formation of important 

 organic systems. It is plain, moreover, that at this stage the 

 animal pole is anterior, the blastopore posterior, and dorsal 

 and ventral surfaces and right and left sides are defined. 

 The conversion of the radial gastrula into the bilaterally 

 symmetrical state may be distinguished by using the term 

 postgastrula. 



Neuropore First coel. seg. 



Neurenteric canal 



FIG. 70. Amphioxus. Lateral views of embryos with three and 

 nine coelomic segments. After Hatschek. 



The flattened dorsal ciliated area is the primitive neural 

 plate, and the correspondingly flattened endoderm below is 

 destined to be the anterior end of the notochord. Growth 

 continues by the multiplication of cells at the blastopore rim, 

 and the postgastrula gradually increases in length, and the 

 areas mentioned are extended along with the rest of the 

 ectoderm and the endoderm ; and as the embryo increases in 

 length the organs are formed from the layers in front. In 

 other words, cells are being constantly formed posteriorly and 

 are differentiated in regular order in front. 



The ectoderm is resolved into a dorsal neural plate con- 

 tinuous in front and laterally with the general ectoderm, and 

 all this ectoderm is being gradually extended by the growth 



