410 THE ORIGIN OF VERTEBRATES 
outgrowths from the metapleure on each side. This canal then 
extends dorsalwards on each side, and so forms the atrial cavity; the 
metapleure still remains in the adult; the somatic muscles in the 
epipleure of the adult are the original body-muscles, and not exten- 
sions into an epipleuric fold, for there is no such fold. 
This explanation is a possible conception for the post-branchial 
portion of the atrium, but is impossible for the branchial region ; for, 
as Macbride points out, as must necessarily be the case, the point of 
origin of the atrial wall is, in all stages of development, situated at; 
the end of the gill-slit. It shifts in position with the position of the 
gill-slit, but there can be no backwards extension of the cavity. 
Macbride therefore agrees with Kowalewsky that the atrial cavity is 
formed by the simultaneous ventral extension of pleural folds, and of 
the branchial part of the original pharynx. Thus, in his summing up, 
he states: “In the larva practically the whole sides and dorsal 
portion of the pharynx represent merely the hyper-pharyngeal groove 
and the adjacent epithelium of the pharynx of the adult, the whole 
of the branchial epithelium of the adult being represented by a very 
narrow strip of the ventral wall of the pharynx of the larva. The 
subsequent disproportionate growth of this part of the pharynx of 
the larva, and of the adjacent portion of the atrial cavity, has given 
the impression that the atrial cavity grew upwards and displaced 
other structures, which is not the case.” 
Further, van Wijhe states that the atrium extends beyond the 
atriopore right up to the anus, just as must have been the case if the 
pleural folds originally existed along the whole length of the body. 
His words are: “ Allerdings hat sich das Atrium beim Amphioxus 
lanceolatus eigenthiimlich ausgebildet, indem sich dasselbe durch 
den ganzen Rumpf bis an den Anus, d.h. bis an die Wurzel des 
Schwanzes ausdehnt.” 
We get, therefore, this conception of the origin of the somatic 
musculature of the vertebrate. The invertebrate ancestor possessed 
on each side, along the whole length of its body, a lateral fold or 
pleuron which was segmented with the body, and capable of move- 
ment with the body, because the dorsal longitudinal somatic muscles 
extended segmentally into each segment of the pleuron. By the 
ventral extension of these pleural folds, not only was the smooth 
body-surface of the vertebrate attained, but also the original appen- 
dages obliterated as such, leaving only as signs of their existence the 
