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- 

 phural 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-airial ridye 

 (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 iirst quite narrow, but gradually extends upwards on each side 

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



4 It 



Fio. 755. — ^Ampbioxus lanceolatus. Ventral aspect of three laryee showing the develop- 

 ment of the atrium, ap. atrlopore ; h, glU-sllts ; //, left metapleural fold ; m. mouth ; rf. right 

 metapleural fold ; w. pre-oral pit. (From Korachelt 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, y?), 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 



