XIII 



PHYLUM CHORDATA 



G3 



laiiK 

 b 



;i, the origina 



econiing tliL^ 



bars of tissue between the iiiidix idcd slits 

 primary lamella'. 



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

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

 jiUiiral 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 suh-atrial rid<jc 

 (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-w'all. This canal is the commencement of the atrium : it is 

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

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



ap 



Fio. 755. — Amphioxus lanceolatus. Ventral aspect of three larv» sliowing the develop- 

 ment of the iitruun. «/). atriopore ; k, gill-slits ; If, left metapleural fold ; m. mouth ; rf. right 

 metapleiiral fold ; w. pre-oral pit. (From Korschelt and Heider, after Ijankester 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 stomodasum. The endostyle appears on the right of 

 the pharynx (Fig. 754,/), 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 



