TRANSACTIONS OF SECTION I. 969 
process extends gradually to the oral end, until a hollow tube is formed connecting 
the mouth with the intestine. In this way the new gut of the adult Petromyzon 
is formed from a solid median rod of cells closely resembling in its formation the 
original notochord. 
I put it forward therefore as a suggestion, that in the ancient times when the 
Merostomata were lords of creation and the competition was keen among these 
ancient arthropod forms, in which the nervous system was so arranged that 
increase of brain substance tended more and more to compress the food channel, 
and therefore to compel to the suction of liquid food instead of the mastication 
of solid, accessory digestive apparatuses were formed, partly in connection 
with the formation of the oral and respiratory chambers, and partly by means of 
the formation of the notochord. Of these accessory methods of digestion the 
former became permanent, while the latter becoming filled up with the peculiar 
notochordal tissue became a supporting structure, still showing by its unsegmented 
character its original function. That a tube formed from the external surface 
either as notochord or as the respiratory portion of the alimentary canal in 
Ammoccetes should be capable of acting as a digestive tube is clear from the 
researches of Miss Alcock,’ for she has shown that the secretion of the skin of 
Ammoceetes easily digests fibrin in the presence of acid. Such a secretion, like the 
similar secretion of the carapace of Daphnia and other crustaceans, was originally 
for the purpose of keeping the skin clean. 
The evidence which I have put before you is in agreement with the conclusion 
that the fore gut of the vertebrate arose gradually from a chamber formed by the 
lamellar branchial appendages, which functioned also as a digestive chamber. By 
the growth of the lower lip, or metastoma, and the modification of the basal portion 
of the last locomotor appendage, which basal part was inside the lower lip, into a 
valvular arrangement like the velum, the animal was able to close the opening into 
the respiratory chamber and feed as blood-sucker in the way of the rest of its kind, 
or, when living food was scarce, keep itself alive by the organic material taken into 
its respiratory chamber with the muddy water in which it lived. 
The Possible Formation of the Vertebrate Spinal Region. 
It remains to briefly indicate the evidence as to the formation of the rest of the 
alimentary canal and the spinal region of the body. 
The problems connected with the formation of this region are of a different 
nature from those already considered in connection with the cranial region. 
In the cranial region the variation that has taken place within the verte- 
brate group and in the course of the formation of the vertebrate is, on the 
whole, of the nature called by Bateson substantive, ze. increase or suppression 
ef parts, while throughout the parts remain constant in their relations to each 
other. It matters not whether it is frog, fish, bird, or mammal we are considering ; 
we always find the same cranial nerves supplying the same segments. When we 
consider the spinal cord and its immediate junction with the cranial region, this 
is no longer so; here we find a repetition of similar segments, with great variation 
in the amount of that repetition ; here we find the characteristic feature is meristic 
variation rather than substantive, and so indetermined is the vertebrate in this 
respect that even now the same species of animal varies in the number of its 
segments and in the arrangement of its nerves. In this part of the vertebrate 
body this repetition is seen not only in the central nervous system and its nerves, 
but also in the excretory organs, so that embryology teaches us that the vertebrate 
body has grown in length by a series of repetitions of similar segments formed 
between the head end and the tail end; such lengthening by repetition of segments 
has been accompanied by the elongation of the unsegmented gut, of the unsegmented 
notochord, and of the unsegmented neural canal, 
To put it shortly, all the evidence points to and confirms the view so strongly 
urged by Gegenbauer, that the head region is the oldest part and the spinal 
: ' Alcock, Proc. Camb. Phil. Soc. vol. vii. 1891, 
1896. 3R 
