406 THE ORIGIN OF VERTEBRATES 



"The hindgut is smaller than the midgut; its anterior limit is 

 marked by the termination of the spiral valve, which does not extend 

 into this region. The two segmental ducts open into it just where it 

 turns ventrally to open to the exterior by a median ventral anus. 

 Its lumen is from an early stage lined with cells which have lost 

 their yolk, and it is in wide communication with the exterior from 

 the first. This condition seems to be, as Scott suggests, connected 

 with the openings of the ducts of the pronephros, for this gland is 

 completed and seems capable of functioning long before any food 

 could find its way through the midgut, or, indeed, before the stomo- 

 dieum has opened." 



Is there no significance in this statement of Shipley ? Even if it 

 be possible to find some special reason why the branchial and cloacal 

 parts of the gut are freed from yolk and lined with serviceable 

 epithelium a long time before the midgut, why should a bit of the 

 midgut, which Shipley calls the oesophagus, which is connected with 

 the region of the pronephros and not of the branchiae, differ so 

 markedly from the rest of the midgut ? Surely the reason is that 

 the branchial region of the gut, the pronephric region of the gut, and 

 the cloacal region of the gut, belong to a different and earlier phase 

 in the phylogenetic history of the Ammoccetes than does the midgut 

 between the pronephric and cloacal regions. This observation of 

 Shipley fits in with and emphasizes the view that the original animal 

 from which the vertebrate arose consisted of a cephalic and branchial 

 region, followed by a pronephric and cloacal region ; the whole inter- 

 mediate part of the gut, which forms the midgut, with its large lumen 

 and spiral valve, and belongs to the mesonephric region, being a later 

 formation brought about by the necessity of increasing the length of 

 the body. 



The OrjGiN of the Somatic Tkunk-Musculature and the 



FORMATION OF AX ATRIAL CAVITY. 



Next comes the question, why was the pronephros not repeated 

 in the meristic repetition that took place during the early vertebrate 

 stage ? What, iu fact, caused the disappearance of the metasomatic 

 appendages, and the formation of the smooth body-surface of the fish ? 



The embryological evidence given by van Wijhe and others of 

 the manner in which the original superficially situated pronephros is 



