xin PHYLUM CHORDATA 63 



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- 

 pleiwal folds (Fig. 755, If. rf., Fig. 756, 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 ridyc 

 (Fig. 756, A, si) 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, bat gradually extends upwards on each side 

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



ap 



FIG. 755. Amphipxus lanceolatus. Ventral aspect of three larvae showing the develop- 

 ment of the atrium, ap. atriopore ; k, gill-slits ; //; left metapleufal fold ; m. mouth ; rf. right 

 metapleural fold ; w. pre-oral pit. (From Korschelt and Heider, after Lankester and Willey.) 



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. 755, B and C), and 

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

 not directly on the exterior, but into a cavity formed by the union 

 of paired ridges of the body-wall, and therefore lined by 

 ectoderm. 



The mouth gradually passes to the ventral surface, and under- 

 goes a relative diminution in size : a fold of integument develops 

 round it and forms the oral hood, which is probably to be looked 

 upon as a stomodseum. The endostyle appears on the right of 

 the pharynx (Fig. 754, fl\ and is at first rod-shaped, then V-shaped : 

 ultimately the limbs of the V unite in the middle ventral line. 

 The gill-slits increase in number and become more and more 

 vertically elongated. The provisional caudal fin disappears. 

 The gonads arise from the outer and ventral regions of the 



