154 THE ORIGIN OF VERTEBRATES 



groups of animals, but that even in the same animal great variations 

 are found, especially in the manner of formation of the limb-plexuses. 

 Such marked meristic variation in the spinal nerves, in contrast to 

 the fixed character of the cranial nerves, certainly points to a more 

 recent formation of the former nerves. 



Also the observations of Assheton on the primitive streak of the 

 rabbit, and on the growth in length of the frog embryo, have led 

 him to the conclusion that, as in the rabbit so in the frog, there 

 is evidence to show that the embryo is derived from two definite 

 centres of growth : the first, phylogenetically the oldest, being a 

 protoplasmic activity, which gives rise to the anterior end of the 

 embryo ; the second, one which gives rise to the growth in length of 

 the embryo. This secondary area of proliferation coincides with the 

 area of the primitive streak, and he has shown, in a subsequent 

 paper, by means of the insertion of sable hairs into the unincubated 

 blastoderm of the chick, that a hair inserted into the centre of the 

 blastoderm appears at the anterior end of the primitive streak, and 

 subsequently is found at the level of the most anterior pair of somites. 



He then goes on to say — 



"From these specimens it seems clear that all those parts in 

 front of the first pair of mesoblastic somites — that is to say, the 

 heart, the brain and medulla oblongata, the olfactory, optic, auditory 

 organs and foregut — are developed from that portion of the un- 

 incubated blastoderm which lies anterior to the centre of the blasto- 

 derm, and that all the rest of the embryo is formed by the activity 

 of the primitive streak area." 



In other words, the secondary area of growth, i.e. the primitive 

 streak area, includes the whole of the spinal cord region, while the 

 older primary centre of growth is coincident with the cranial region. 



In searching, then, for the origin of the segmental nerves, we 

 must consider the type on which the cranial nerves are arranged 

 rather than that of the spinal nerves. 



The first striking fact occurs at the spino-occipital region, where 

 the spinal cord merges into the medulla oblongata, for here in the 

 cervical region we find each spinal segment gives origin to three dis- 

 tinct roots, not two — a dorsal root, a ventral root, and a lateral root. 

 This third root gives origin to the spinal accessory nerve, and in the 

 region of the medulla oblongata these lateral roots merge directly 

 into the roots of the vagus nerve; more anteriorly the same system 



