56 ZOOLOGY SECT. 



produced in a single longitudinal series. Above, i.e. dorsal to 

 them, a second longitudinal series makes its appearance, containing 

 eight clefts, so that at this stage there are two parallel rows of 

 gill-slits on the right side of the body, and none on the left. But 

 as growth goes on, the first or ventral series gradually travels over 

 to the left side, producing a symmetrical arrangement, and at the 

 same time the first slit and the last five of the first or definitively 

 left series close up and disappear, so that the numbers are 

 equalised on the two sides. At first each gill-slit is simple, but 

 before long a fold grows down from its dorsal edge, and, proceeding 

 ventrally, divides the single aperture into two : this fold is the 

 secondary or tongue lamella, the original bars of tissue between 

 the undivided slits becoming the primary lamellae. 



While the development of the gill-slits is proceeding, the atrium 

 is in course of formation. Paired longitudinal ridges, the meta- 



ap 



FIG. 710. AmphioxUS lanceolatus. Ventral aspect of three larvae showing the develop- 

 ment of the atrium, up. atriopore ; A-, gill-slits ; //'. left metapleural fold ; m. mouth ; rf. right 

 metapleural fold ; ?', prae-oral pit. (From Korschelt and Heider, after Lankester and Willey.) 



pleural folds (Fig. 710, If. rf., Fig. 711, sf.) appear on the ventral 

 side of the body, behind the gill-slits, and gradually extend for- 

 wards, dorsal to the latter, their arrangement being very unsym- 

 metrical in correspondence with that of the clefts themselves. 

 On the inner face of each fold, i.e. the face which looks towards 

 its fellow of the opposite side, a longitudinal sub-atrial ridge 

 (Fig. 711, A, 67) appears, and, the two sub-atrial ridges meeting and 

 coalescing, a canal (B, p) is formed immediately below the ventral 

 body-wall. This canal is the commencement of the atrium : it is 

 at first quite narrow, but gradually extends upwards on each side 

 (C, pi) until it attains its full dimensions. It is open, at first, both 

 in front and behind : the posterior opening remains as the atrio- 

 pore : the anterior opening becomes gradually shifted forwards as 

 the fusion of the sub-atrial ridges proceeds (Fig. 710, B and C), and 

 is finally completely closed. In this way the gill-slits come to open, 



