412 THE ORIGIN OF VERTEBRATES 
desire to emphasize what appears to me to be the fact, that the 
musculature, which in the region of the trunk would correspond to 
that derived from the ventral segmentation of the mesoblast in the 
region of the head, may have arisen not only from the musculature of 
the appendages, but also from the ventral longitudinal musculature 
of the body of the invertebrate ancestor, for it seems probable that 
this latter musculature had nothing to do with the origin of the great 
longitudinal muscles of the vertebrate body, either dorsal or ventral. 
The way in which I imagine the obliteration of the atrial cavity 
to have taken place is indicated in Fig. 160, B, which is a modifica- 
tion of a section across a trilobite-like animal as represented in 
Fig. 160, A. As is seen, the pleural folds on each side have nearly 
met the bulged-out ventral body-surface. A continuation of the 
same process would give Fig. 160, C, which is, to all intents and 
purposes, the same as Fig. 159, C, taken from van Wijhe, and shows 
how the segmental duct is left in the remains of the atrial cavity. 
The lining walls of the atrial cavity are represented very black, in 
order to indicate the presence of pigment, as indeed is seen in the 
corresponding position in Ammoccetes. In these diagrams I have 
represented the median ventral surface as a large bulged-out bag, 
without indicating any structures in it except the ventral extension 
of the proccelom to form the metacceelom. At present I will leave 
the space between the central nervous system and the ventral mesen- 
tery blank, as in the diagrams; in my next chapter I will discuss 
the possible method of formation within this blank space of the 
notochord and midgut. Boveri considers that the obliteration of the 
atrial cavity in the higher vertebrates is not complete, but that its 
presence is still visible in the shape of the pronephric duct. The 
evidence of Maas and others that the duct is formed by the fusion 
of the pronephric tubules is, it seems to me, conclusive against 
Boveri's view ; but yet, as may be seen from my diagrammatic figures, 
the very place where one would expect to find the last remnant 
of the atrial cavity is exactly where the pronephric duct is situated. 
For my own part I should expect to find evidence of a former 
existence of an atrial cavity rather in the pigment round the prone- 
phros and its duct than in the duct itself. 
The conception that Amphioxus shows us how to account for the 
great envelope of somatic muscles which wraps round the vertebrate 
body, in that the ancestor of the vertebrate possessed on each side of 
