136 CEPHALOCHORDATA CHAP. 
growth towards each other of sub-atrial ridges on the inner sides 
of the metapleural folds (see Fig. 89, A, sl), 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 le outside the gill-slits (Fig. 88, A) when these folds close 
i 
nT 
TOSS? sees ct 
ZIM 
Fic. 89.—Diagrammatic transverse sections of three larvae of Amphioxus to show the 
development of the atrium. ao, Aorta; ce, dermis; ch, notochord ; d, intestine ; 
J, connective tissue; fh, cavity of dorsal fin-ray ; m, myotome; m, nerve-tube ; 
p, atrium ; sf, metapleural folds ; s/h, lymph space in metapleural folds ; si, sub- 
intestinal vein ; sk, sheaths of notochord and nerve-tube ; s/, 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 
