THE NOTOCHORD AND ALIMENTARY CANAL 45 1 



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 tiring must, I 

 think, have occurred. The grooved ventral surface became a tube, 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 oesophagus, 

 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 



