THE REGION OF THE SPINAL CORD 409 
dorsal segmented portion, the protovertebra, and a ventral unseg- 
mented portion, the lateral plates. He describes in the dorsal part 
the formation of myotome and sclerotome, as in the Craniota. 
Also, he describes how the myotome is at first confined to the dorsal 
region in the neighbourhood of the spinal cord and notochord, and 
subsequently extends ventrally, until, just as in Ammoceetes, the 
body is enveloped in a sheet of somatic segmented muscles, the well- 
known myomeres. 
The conclusion to be drawn from this isinevitable. Any explana- 
tion of the origin of the somatic muscles in Ammoccetes must also 
be an explanation of the somatic muscles in Amphioxus, and con- 
versely ; so that if in this respect Amphioxus is the more primitive 
and simpler, then the condition in Ammoccetes must be looked upon 
as derived from a more primitive condition, similar to that found in 
Amphioxus. Now, it is well know that a most important distinction 
exists between Amphioxus and Ammoccetes in the topographical 
relation of the ventral portion of this muscle-sheet, for in the former 
it is separated from the gut and the body-cavity by the atrial space, 
while in the latter there is no such space. Furbringer therefore 
concludes, as I have already mentioned, that this space has become 
obliterated in the Craniota, but that it must be taken into considera- 
tion in any attempt at formulating the nature of the ancestors of the 
vertebrate. 
Kowalewsky described this atrial space as formed by the ventral 
downgrowth of pleural folds on each side of the body, which met in 
the mid-ventral line and enclosed the branchial portion of the gut. 
According to this explanation, the whole ventral portion of the 
somatic musculature of the adult Amphioxus belongs to the extension 
of the pleural folds, the original body-musculature being confined to 
the dorsal region, This is expressed roughly on the external surface 
of Amphioxus by the direction of the connective tissue septa between 
the myotomes (cf. Fig. 162, B). These septa, as is well known, bend 
at an angle, the apex of which points towards the head. The part 
dorsal to the bend represents the part of the muscle belonging to the 
original body; the part ventral to the bend is the pleural part, and 
represents the extension into the pleural folds. 
Lankester and Willey have attempted to give another explanation 
of the formation of the atrial cavity ; they look upon it as originating 
from a ventral groove, which becomes a canal by the meeting of two 
