4IO THE ORIGIN OF VERTEBRATES 



outgrowths from the metapleure on each side. This canal then 

 extends dorsal wards 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 Ampliloxus 

 lanccolatus eigenthumlich ausgebildet, indem sich dasselbe durch 

 den ganzen Eumpf 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 



