THE NOTOCHORD AND ALIMENTARY CANAL 441 
notochord blind, just as it had already left the end of the infundi- 
bulum blind. 
The whole evidence points to the derivation of the notochord 
from a ventral groove on the surface of the animal, which closed 
to form a tube capable of acting as an accessory gut at the critical 
period before the new gut was fully formed. The essentials of a gut 
tube are absorption and digestion of food; is it likely that a tube 
formed as I have suggested would be efficient for such purposes ? 
As far as absorption is concerned, no difficulty would arise. 
The gut of the arthropod is lined with a thin layer of chitin, 
which is traversed, like all other chitinous surfaces, by fine canali- 
culi. Through these canaliculi, absorption of fluid material takes 
place, from the gut to the body. Similar canaliculi occur in the 
chitin covering the animal externally, so that, if such external 
surface formed a tube, and food in the right condition for absorption 
passed along it, absorption could easily take place through the 
chitinous surface. The evidence of Apus proves that food does 
pass along such a tube in the open condition, and in the trilobites 
the chitinous surface lining a similar groove was apparently very thin, 
a condition still more favourable to such an absorption process. 
At first sight the second essential of a gut-tube—the power 
of digestion—appears to present an insuperable difficulty to this 
method of forming an accessory gut-tube, for it necessitates the for- 
mation of a secretion capable of digesting proteid material by the 
external cells of the body, whereas until recently it was supposed 
that such a function was confined to cells belonging to the so-called 
hypoblastic layer. Experiments were made now years ago of 
turning a Hydra inside out so that its internal layer should become 
external, and vice versd, and they were said to have been successful. 
Such an animal could go on living and absorbing and digesting food, 
although its epiblastic surface was now its digestive internal surface. 
More recent observations have shown that these experiments were 
fallacious. At night-time, when the observer was not looking, the 
hydra reinverted itself, so that again its original digestive surface 
was inside and it lived and prospered as before. 
Another piece of evidence of somewhat similar kind, which has 
not as yet been discredited, is seen in the Tunicata. In many of 
these, new individuals are formed from the parent by a process of 
budding, and it has been proved that frequently the gut of the new 
