136 



CEPHALOCHORDATA 



growth towards each other of sub-atrial ridges on the inner sides 

 of the metapleural folds (see Fig. 89, A, si'), and then becomes 

 greatly enlarged by the increased relative growth of the ventro- 

 lateral part of the body -wall (Fig. 89, B, C). The posterior 

 opening between the metapleural folds remains as the atriopore 

 (Fig. 88, C, ap); while the anterior end (Fig. 88) also remains 

 open for some time, but eventually closes. As the metapleural 

 folds lie outside the gill-slits (Fig. 88, A) when these folds close 



FIG. 89. Diagrammatic transverse sections of three larvae of Amphioxus to show the 

 development of the atrium, ao, Aorta ; c, dermis ; ch, notochord ; d, intestine ; 

 f, connective tissue ; fh, cavity of dorsal fin-ray ; m, myotome ; n, nerve-tube ; 

 p, atrium ; sf, metapleural folds ; sfh, lymph space in metapleural folds ; si, sub- 

 intestinal vein ; sk, sheaths of notochord and nerve-tube ; si, sub-atrial ridge ; sp, 

 coelom. (From Korschelt and Heider, after Lankester and Willey.) 



in (B and C), it comes about that the gill-slits which formerly 

 opened freely to the exterior now open into the cavity of the 

 atrium (compare Figs. 87 and 88). 



The mouth now becomes median and ventral, and is reduced 

 in size, the oral hood (stomodaeum) is formed in front of it, the 

 gill-slits become more numerous and vertically elongated, the 

 endostyle forms along the floor of the pharynx, and the gonads 

 grow as paired pouches from the body-wall. This brings the 

 animal to the young adult condition, reached at a period of prob- 

 ably about three months after the fertilisation of the egg. 



The development as a whole shows a very marked resemblance 



