444 THE ORIGIN OF VERTEBRATES 
roots. Lwoff and Rolph both describe a direct communication 
between the spinal cord and the notochord by means of fibres 
passing through these holes, without however looking upon this con- 
nection as a nervous one. Joseph alone asserts that no absolute 
connection exists, for the internal elastic layer of the notochord, 
according to him, is not interrupted at these holes, and forms, therefore, 
a barrier between the fibres from the spinal cord and those from the 
interior of the notochord. Still, whatever is the ultimate verdict as 
to these fibres, the suggestive fact remains of the spaces in the 
notochordal sheath and of the corresponding projecting root-like fibres 
from the spinal cord. The whole appearance gives the impression of 
some former connection, or rather series of connections, between the 
spinal cord and the notochord, such as would have occurred if nerves 
had once passed into the notochord. On the other hand, such nerves 
were not arranged segmentally with the myotomes, for, according to 
Joseph, in the middle of the animal ten to twelve such holes occur in 
one body-segment. In Apus the appendages are more numerous than 
the body-segments, so that it is not necessary for a segmental arrange- 
ment to coincide with that of the body-segments. z 
THE ORIGIN OF THE ALIMENTARY CANAL. 
In close connection with the notochord is the alimentary canal. 
Any explanation of the one must be of assistance in explaining 
the other. 
According to the prevalent embryological teaching, the body is 
formed of three layers, epiblast, hypoblast, and mesoblast, and the 
gastreea theory of the origin of all Metazoa implies of necessity that 
the formation of every individual commences with the formation of 
the gut. For this reason the alimentary canal must in every case 
be regarded as the earliest formed organ, however late in the develop- 
ment it may attain its finished appearance. Hence the notochord is 
spoken of as developed from the mid-dorsal wall of the alimentary 
canal, It is possible to look at the question the other way round, 
and suppose that the organ whose development is finished first is 
older than the one still in process of making. In this case it would 
be more right to say a ventral extension of the tissue, which gives 
rise to the notochord, takes place and forms the alimentary canal. 
It is, to my mind, perfectly possible, and indeed probable, that 
