270 



Basic Structure of Vertebrates 



Just as the mesodermal pouches arise in sequence from anterior to 

 posterior, their differentiation also proceeds anteroposteriorly. Those 

 of the first pair may have progressed to the point of differentiating 

 muscle while as yet only 10 of the eventual 61 pairs have been 

 definitely formed. 



An embryo of Amphioxus, at a stage when 14 or 15 pairs of meso- 

 dermal pouches are present, is a delicate, colorless, transparent animal 

 having a length of about 1 mm. and a diameter of one-eighth that 

 except at the somewhat enlarged anterior end (Fig. 224). It has a 

 straight digestive tube (enteron, I) extending from an anterior mouth 

 to a posterior anus. There is a single gill-cleft, opening from the right 

 side of the anterior region of the digestive tube. The mouth also is 

 unsymmetric at this stage, opening on the left side. Later, as numerous 

 additional gill-clefts are formed, they shift their positions so as to be- 

 come ultimately a series of symmetrically placed paired apertures. 

 Meanwhile the mouth shifts from its original left to a median position. 

 Just above the digestive tube lies the median rodlike notochord (NC), 

 extending the entire length of the animal. Immediately above the noto- 

 chord is the neural tube (NT), its somewhat enlarged anterior region 

 suggesting a brain. At the anterior end of the neural tube, the dorsal 

 neuropore (NP) is still open. The neurenteric canal (NE), at this stage, 

 has ordinarily become closed. In the anterior region, where the differ- 

 entiation of the mesoderm is most advanced, a coelom intervenes be- 

 tween the enteric tube and the outer body-wall (Fig. 223, C). The 

 body-wall (somatopleure) consists of the ectoderm and the somatic 

 layer of mesoderm. The enteric endoderm, together with the contigu- 

 ous visceral or splanchnic layer of mesoderm, constitutes the wall 

 (splanchnopleure) of the digestive tube. The somatic and visceral 

 sheets of mesoderm provide the coelom with a continuous and com- 

 plete lining, the peritoneum. The superficial ectoderm is a skin. The 



Fig. 224. Amphioxus at beginning of larval period; 14 or 15 pairs of mesodermal 

 somites. Actual length of larva about 1.0 mm. (CG) Club-shaped gland; (I) intes- 

 tine; (MES) mesodermal somites; (NC) notochord; (NE) neurenteric canal; (NP) 

 neuropore; (NT) neural tube; (P) pigment spot in neural tube. (After Hatschek. 

 Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston 

 Company.) 



