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- 
deeum 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 cesophagus, which is connected with 
the region of the pronephros and not of the branchis, 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 pronepbric 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 ORIGIN OF THE SOMATIC TRUNK-MUSCULATURE AND THE 
FORMATION OF AN 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, in 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 
