THE NOTOCHORD AND ALIMENTARY CANAL A5I 
open groove, as already explained for both notochord and the 
anterior part of the gut itself in Ammoccetes; an open groove 
formed from the mid-ventral surface of the body, on each side of 
which were the remnants of the pronephric appendages. By the 
closure of this groove ventrally, and the growing round of the pleural 
folds, as already suggested, the remains of the pronephric appendages 
are indicated by the segmental duct and the form of the vertebrate 
body is attained. 
Even in the branchial region the same kind of thing must, I 
think, have occurred. The grooved ventral surface became a tybe, on 
each side of which were lying in regular order the in-sunk branchial 
appendages, the whole being subsequently covered by the pleural 
folds to form an atrial chamber. A tube thus formed from the 
grooved ventral surface would carry with it to the new ventral 
surface the longitudinal venous sinuses, and thus form, in the way 
already suggested, the heart and ventral aorta. Posterior to the 
heart in the pronephric region, the same process would give rise to 
the sub-intestinal vein. 
The evidence of comparative anatomy bears out most con- 
clusively the suggestion that in the original vertebrate the gut was 
mainly a respiratory chamber. In man and all mammals the oral 
chamber opens into a small pharynx, followed by the cesophagus, 
stomach and small intestine. Of this whole length, a very small 
part is taken up by the pharynx, in which, in the embryo, the 
branchial arches are found, showing that this represents the original 
respiratory part of the gut. In the ordinary fish this branchial part 
is much more conspicuous, occupies a large proportion of the gut, 
and in the lowest fishes, such as Ammoccetes and Amphioxus, the 
branchial region extends over a large portion of the animal, while 
the intestine proper is a straight tube, the length of which is in- 
significant in comparison with its length in the higher vertebrates. 
Such a tube was able to act as a digestive tube, owing, as already 
pointed out, to the digestive powers of the skin-epithelium, and I 
imagine at first the respiratory chamber, seeing that it composed 
very nearly the whole of the gut, was at the same time the main 
digestive ‘chamber; even in Ammoccetes its digestive power is 
superior to that of the intestine itself. 
Just posterior to the branchial part a diverticulum of the gut was 
formed at an early stage, as seen in-Amphioxus, and provided the 
