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Basic Structure of Vertebrates 



Amphioxus 



Fig. 193. Early embryonic stages (if Amphioxus (a chordate) and a sea urchin 

 (an echinoderm). 



(Top) Egg, blastula, and gastrula of Amphioxus. (Bottom) Egg, blastula, gas- 

 trula, and early larval (pluteus) stage of sea urchin. The eggs are of about the 

 same size (diameter 0.1 mm.). The eggs and the pluteus are represented as semi- 

 transparent entire objects. The blastulas and gastrulas are represented in axial 

 section. (A) Archenteron, the prospective digestive cavity; (B) blastocoele; (Bp) 

 blastopore; (Bp-A) blastopore persisting as anus; (EC) ectoderm; (EN) endoderm; 

 (M) mouth; (S) calcareous skeletal rods. 



The gastrula of Amphioxus proceeds directly to develop a neural tube and noto- 

 chord, basic features of a chordate. The gastrula of the sea urchin within a few 

 hours becomes transformed into a pluteus which, in form and structure, is totally 

 devoid of chordate characteristics. 



trula — are common to the great majority of animal embryos. The egg, 

 blastula, and gastrula of a sea urchin would seem to differ in no impor- 

 tant way from the corresponding stages of the embryo of Amphioxus, 

 a small, somewhat fishlike animal sometimes classified as a vertebrate 

 (Fig. 193). However, that animals resemble one another more and 

 more closely the earlier the stages which are compared, is not because 

 points of similarity become more numerous but because points of 

 difference become fewer. By the time the sea urchin embryo has passed 

 over into the characteristic larval form, the pluteus, it has positively 

 declared itself to be a sea urchin. The chick embryo as early as its 

 second day of development has a dorsal neural tube, an unmistakable 

 notochord, and a ventral heart. It is irrevocably a vertebrate. In the 

 foregoing enumeration of basic features of vertebrates, the embryonic 

 stages to which reference has been made have in no case been earlier 

 than a stage in which the general body-plan and the pattern of organs 

 have become so definitely established as to amount to a "declaration 

 of intentions" to become a vertebrate. 



This general body-plan, with its several basic organs, constitutes 



