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 Wijke, 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 metaccelom. At present I will leave 

 the space between the central uervous 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 



