THE DEVELOPMENT OF AMPHIOXUS 49 



the atriopore (Fig. 17). The cavity, at first very small, gradu- 

 ally enlarges (Fig. 18, C) pushing upward each side of the 

 pharynx, ultimately surrounding it on all sides except the 

 dorsal, where alone the pharynx remains connected with the 

 body wall. The space occupied by the atrium was previously 

 the region of the general coelom. The development of the 

 atrium therefore reduces the coelom to a number of greatly 

 reduced and somewhat scattered spaces. The chief of these 

 are (a) the pair of dorsal ccelomic canals along the dorso-lateral 

 regions of the pharynx, from which the atrium remains sepa- 

 rated by the suspensory folds or denticulate ligament; (6) the 

 endostylar ccelom in the ventral region below the endostyle; 

 and (c) the branchial ccelomic canals connecting the two pre- 

 ceding regions by way of very small canals in the primary 

 branchial arches. Posterior to the atriopore the coelom remains 

 for a time in its normal relation, but later the atrial cavity 

 extends into this region on the right side nearly to the anus, 

 leaving the coelom widely open only on the left side of the 

 intestine. 



The atrial cavity is formed in the branchial region just before 

 the primary gill slits (except perhaps the more anterior) are 

 perforated, so that from the first these open into the atrial 

 cavity and never directly to the outside. From its mode of 

 formation the denticulate ligament or suspensory fold is seen 

 to be a membrane equivalent to the body wall, and therefore 

 covered on its atrial surface with ectoderm. Of course the 

 remainder of the atrial cavity is lined with ectoderm, except, 

 as we have seen, that the endodermally covered tongue bars 

 line a small portion of it. 



9. Larval Asymmetry 



The nature and extent of the larval asymmetry of Amphioxus 

 represent the most important difference between the develop- 

 ment of this form and other Chordates. The asymmetry of 

 such structures as the neuropore and anus, which have merely 

 been displaced slightly by a later medially developing structure 



