200 
The way in which we must imagine the conversion of 
the Annelid into the Chordate is represented in fig. 3. 
As in all other theories of the Annelidan origin of Verte- 
brates, here also the ventral surface of the Annelid corresponds 
to the dorsal one of the Vertebrate. One serious objection 
to my theory seems to present itself here: the way in 
„which in ontogeny the medullary tube forms does not exactly 
„answer to what we should have expected after the above 
diagram and in accordance with the law of recapitulation. 
We might expect the neural tube to originate as a tubular 
invagination of the ectoderm, afterwards lengthening very 
much from in front backwards and thus pushing its inner 
end and the former blastopore under the neural body-wall to 
the caudal end of the embryo. This is not the case, as we know. 
In the last chapter, however, we will see that this apparent 
difficulty is not only completely solved, but that the earliest 
ontogenetic processes on the contrary yield the strongest pos- 
sible support to my theory and that only the peculiar way 
in which some of these processes interfere has proved a 
hindrance to their interpretation and delayed for so long a 
time the solution of the problem of the origin of Vertebrates. 
Spinal ganglia. — The great difference between the central 
nervous system of Vertebrates and Protostomia is that in 
the former it originates as a tube, whereas in the latier it 
consists of ganglia, though in both cases it is of ectodermal 
origin. Ganglia, however, are found in Vertebrates also and 
they even play a very important role in the formation of the 
nervous system. We find a pair of them, the spinal ganglia, 
in every segment af the body, just like in Annelids. Also 
the situation and place of origin, the median line of the 
neural body-wall, where the medullary folds have met and 
coalesced, correspond exactly to what we find in Annelids. 
It seems to me fairly evident that, if we are right until 
now, we can hardly doubt the homology of the spinal 
ganglia of Vertebrates and the ventral ganglia of ‘Annelids 
and Arthropods. On the other hand this assumption might 
contribute to make us understand the conversion of the 
stomodaeum into the neural tube. If indeed a strong longi- 
tudinal growth of the stomodaeum had taken place phylo- 
genetically, as represented in fig. 3, it would have come 
to lie along its whole length against the ventral chain of 
ganglia. From these ganglia, nerve-fibres, formerly uniting a 
right and a left ganglion, might have grown into the stomo- 
