36 OUTLINES OF CHORDATE DEVELOPMENT 



fin. Upon the formation of the anus the intestine is separated 

 from the neurenteric canal and this structure closes and gradu- 

 ually moves posteriorly, away from the anal region, as the tail 

 region grows out; but for some time the tip of the nerve 

 cord remains curved around toward the region of its original 

 connection with the enteron. 



About the time the mouth and anus open, the alimentary 

 tract becomes ciliated and a small diverticulum forms in the 

 anterior part of the stomach-intestine; this is the rudiment of 

 the caecum or liver. 



At the close of the embryonic period, which is arbitrarily 

 marked by the opening of the mouth, the embryo is about 

 1.0 mm. long and very slender (Fig. 9, C). Probably its actual 

 bulk is little greater than that of the egg. The embryo is 

 almost perfectly transparent, and swims about with the elon- 

 gated ectodermal cilia, accompanied by irregular muscular con- 

 tractions when strongly stimulated. The anterior end is di- 

 lated and prolonged forward into a rostrum, containing the 

 preoral head cavity and supported by the elongated notochord. 

 The external openings are, the mouth and first gill slit on 

 opposite sides of the head, above these the small median 

 neuropore, and near the posterior end the small ventral anus. 

 Posteriorly the tail is expanded into a provisional caudal fin 

 formed of elongated ectodermal cells. There are about fifteen 

 pairs of somites in every stage of development, from the com- 

 pletely undifferentiated condition of the posterior pairs of the 

 series, to the quite completely differentiated anterior pairs. 

 Anteriorly the nervous system shows the rudiment of the brain; 

 posteriorly it is no longer connected with the enteron on account 

 of the closure of the neurenteric canal. The notochord is com- 

 pletely established. In addition to the mouth and first gill slit 

 the enteron has formed the anterior gut diverticula and club- 

 shaped gland, all of which have become separate structures 

 and commenced their differentiation. In spite of its rather 

 complex structure all parts of the embryo are still of epithelial 

 character, though the epithelia now show some degree of 

 elementary cellular differentiation. 



