. The chordates. Chordata. 228 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 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 pe
. The chordates. Chordata. 228 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 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 o
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