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 Ammoccetes, the 

 body is enveloped in a sheet of somatic segmented muscles, the well- 

 known myomeres. 



The conclusion to be drawn from this is inevitable. Any explana- 

 tion of the origin of the somatic muscles in Ammoccetes must also 

 be an explanation of the somatic muscles in Aniphioxus, and con- 

 versely ; so that if in this respect Aniphioxus 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 

 Aniphioxus. Now, it is well know that a most important distinction 

 exists between Aniphioxus 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. Fiirbringer 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 011 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 Aniphioxus 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 Aniphioxus by the direction of the connective tissue septa between 

 the myotomes (c/. 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 



