REPRODUCTION 



6i 



cavities are all thrown into free communication to form one large space, 

 the definitive coelom, which finally shows no trace of its segmental origin. 

 An embryo of Amphioxus, at a stage when fourteen or fifteen pairs 

 of mesodermal pouches are present, is a delicate, colorless, transparent 

 animal having a length of about one millimeter and a diameter of one- 

 eighth that except at the somewhat enlarged anterior end (Fig. 53). It 

 has a straight digestive tube (enteron, /) 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 unsymmetrical at this stage, opening on the left side. Later, as numer- 

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

 become 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 rod-like notochord (NC) 

 extending the entire length of the animal. Immediately above the 



Fig. 53. — Amphioxus at beginning of larval period; 14 or 15 pairs of mesodermal 

 somites. Actual length of larva about i.o mm. CG, club-shaped gland; /, intestine; 

 MES, mesodermal somites; NC, notochord; NE, neurenteric canal; NP, neuropore; 

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



notochord is the neural tube (A^T), its somewhat enlarged anterior region 

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

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

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

 tiation of the mesoderm is most advanced, a coelom intervenes between 

 the enteric tube and the outer body-wall (Fig. 52, C). The body-wall 

 (somatopleure) consists of the ectoderm and the somatic layer of meso- 

 derm. The enteric endoderm together with the contiguous visceral or 

 splanchnic layer of mesoderm constitute the wall (splanchnopleure) of the 

 digestive tube. The somatic and visceral sheets of mesoderm provide 

 the coelom with a continuous and complete lining, the peritoneum. 

 The superficial ectoderm is a skin. The more anterior myotomes contain 

 partially differentiated muscle tissue capable of feeble contraction. 

 The animal is free-swimming but the locomotor mechanism consists 

 merely of long cilia produced by the ectodermal layer. 



In its main features this young Amphioxus is like a vertebrate. If its 

 true origin and nature were not known, it might reasonably be expected to 



