DEVELOPMENT OF REFLEX MECHANISMS 83 
than in the younger embryos of the first reacting stages. In 
addition to these long paths, short reflex connections are now 
possible within a single segment of the cord. Nevertheless the 
dendrites of both individual correlation neurones and motor 
neurones reach all parts of the white substance, so that what- 
ever the source of the stimulus a common type of total response 
habitually follows, as in the younger specimens—a simple swim- 
ming reflex. 
The mammalian spinal cord shows a much more complete 
differentiation of individual reflex systems and a more perfect 
isolation of the long conduction pathways. 
In the medulla oblongata of the half grown larva the periph- 
eral sensory neurones show a high degree of functional speci- 
ficity, and the central neurones of the second order tend to be 
grouped around these special sensory roots. But the functional 
localization of these secondary centers is not complete, each 
neurone having a dominant relation to some particular system 
of root fibers but also subsidiary connections with other function- 
ally distinct systems of root fibers. Thus each primary bulbar 
center reached by terminals of sensory root fibers is also to some 
extent a correlation center, the secondary tracts are all of mixed 
function, and the analysis of function in the reactions is still 
incomplete. The peripheral sensory neurones, though segre- ~ 
gated into functional systems as distinctly as in the cranial 
nerves of higher vertebrates and much more so than in the spinal 
nerves of any forms, are similar to those of the spinal nerves in 
that each fiber centrally reaches practically the entire length of 
the oblongata, giving to the primary sensory centers a longi- 
tudinal columnar arrangement which is apparently much more 
primitive than the more condensed sensory nuclei of mammals. 
In the mammals the functional differentiation of the primary 
bulbar centers is complete and the functions of correlation are 
transferred to higher cerebral centers. Simpler total reactions 
of the more primitive sort are, however, still provided for in the 
reticular formation of the oblongata in these higher brains. 
Throughout this series of forms of reflex connections we find 
a progressive differentiation of the specific reflexes away from, the 
