54 OUTLINES OF CHORDATE DEVELOPMENT 



when four or five slits are formed. When the anterior and 

 posterior primary gill slits atrophy the associated nephridia 

 seem also to atrophy and later to be re-formed in connection 

 with the tertiary gill slits although actual details in the pro- 

 cess are unknown. 



e -=, 



12. The Larva at the Critical Stage 



We may now summarize briefly the most important char- 

 acters of the larva at the close of this period, in what is called 



the critical stage, marked by the sym- 

 metrization of the mouth and pharynx, 

 and the reduction of the gill slits to eight 

 symmetrical pairs. The larva (Fig. 15, B} 

 is about 3.5 mm. in length and tapered 

 at each extremity. The only exter- 

 nal opening added during this period is 

 the atriopore, and the neuropore is about 

 to close. The mouth has assumed a 

 symmetrical anterior position, and in 

 front of it a buccal cavity has been es- 

 tablished by the outgrowth of the oral 

 hood, from the base of which the buc- 

 cal cirri have begun to grow out. The 

 f^j num ber (61) of somites has been 



' m 



larva pass- formed for some time, and a definite tail 



ing through the external v 



(atrial) opening of a nep- re S 1On has g^' 11 Ollt posterior to the 



hridium. After Good- ana l opening. The provisional caudal 



rich, c, Ccelom; e, bran- _ 



chial epithelium; g, wall nil has been replaced by a permanent 



f^^^SZS caudal fi fold continuing anteriorly as 

 space; n, wall of nephri- the median dorsal and ventral fins, the 



dium; o, atrial (external) ,, ,. .,, . , , 



opening of nephridium. lormer continuous with the right side of 



the oral hood. The ventral fin has dis- 

 placed the anus to the left side. The atrium is a spacious 

 cavity receiving the external openings of the eight pairs of 

 gill slits, of which the anterior pair remains undivided by a 

 tongue bar, while the posterior one or two pairs are as yet in- 



m 



FIG. 20.-Portion of 



a transverse section 01 



