'14 THE ANCESTRY OF VERTEBRATES 



The way in which we must imagine the conversion of 

 the Annelid into the Choi date 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 lengtnening 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 latter 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, ii would have come 

 to lie along its whole length against the ventral chain of 

 ganglia. From these ganglia, nerve-fibres, formerly uniting a 



jright and a left ganglion, might have grown into the stomo- 



