68 ORIGIN OF CHORDATES in. 5- 



of a chordate plan of organization seen in the adult. The result of 

 development is to produce a fish-like creature, the ascidian tadpole, 

 which is immediately recognizable as a chordate (Fig. 37). The 

 cleavage is total and produces a blastula with few cells, whose future 



mus.c. 



atr. 



mus. c. 



Fig. 38. The ascidian tadpole (Ascidia or Ciona type). 1. Tadpole ready to hatch. 

 2. Tadpole. 3. Sensory vesicle. 4. Cross section of tail. 



atr. atrium; end. endostyle; fol. follicle cells; mus.c. muscle cells; mus.f. muscle fibrils; 



n.c. nerve-cord; n.m. nerve to tail muscles; not. notochord; oc. ocellus; ot. otolith; su. sticking 



gland; ves. sensory vesicle. (After Berrill.) 



potentialities are already determined. Gastrulation by invagination 

 follows and the creature then proceeds to elongate into the fish-like 

 larva. This possesses an oval 'head' and long tail, the latter supported 

 by a notochord formed by cells derived from the archenteric wall. 

 Forty of these cells make up the entire rod, becoming vacuolated and 

 elongated by swelling. 



On either side of this notochord run three rows of muscle-cells, 

 eighteen on each side, derived from mesoderm that arises from yellow- 

 pigmented material already visible in the egg and later forming part 

 of the wall of the archenteron. Other cells of this tissue migrate 

 ventrally to make the pericardium, heart, and mesenchyme. The 



