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Cruce 
sparseness of terminals and to inappropriate 
location (laminae V and VI). In summary, it 
seems likely that descending control of limbs 
is not highly organized except in those ani- 
mals, such as mammals, where the cortico- 
spinal and corticorubrospinal pathways have 
become highly developed. 
Evolution of Motor Systems 
Although only one reptilian suborder, liz- 
ards, has been studied in detail in this labor- 
atory and although only scant information 
is available regarding other reptiles (Donke- 
laar, 1976a, b) and other nonmammalian 
vertebrate classes, there are other hypo- 
theses regarding the evolution of motor sys- 
tems worthy of investigation because of the 
experiments that they may generate. 
(1) The location of cell bodies in the spinal 
cord has not changed dramatically from rep- 
tiles to mammals and may, indeed, be similar 
in all tetrapods. What has changed is the 
orientation and extent of the dendritic 
branching pattern of spinal motoneurons; 
the dendritic morphology of other spinal 
neurons is less understood and may not have 
changed much. Consistent with this hypothe- 
sis, frog motoneurons (Szekely, 1976) show 
a pattern of dendritic branching similar to 
that of lizard motoneurons, but with even 
more small branches in the white matter. 
(2) Each vertebrate class may be char- 
acterized by one major descending motor 
pathway which it lacks or possesses when 
compared to other classes, and there may be 
an hierarchical sequence in which pathways 
were added as the classes evolved. Birds 
which evolved in parallel with mammals 
appear to possess a small projection from 
the forebrain to the brainstem and rostral 
spinal cord (Nauta and Karten, 1970) and 
this may be comparable to mammalian cor- 
ticospinal and corticobulbar pathways. 
Although reptiles lack the corticospinal path- 
way, all of the bulbospinal pathways found 
in mammals are present. One of the bulbo- 
spinal pathways, the rubrospinal, is believed 
to exi.st in amphibia (Ariens Kappers et al.. 
1936) and it also appears to be absent or 
greatly reduced in one snake species which 
has been studied (Donkelaar, 1976a, b). 
Less is known about other vertebrate classes, 
but the medullary reticulospinal and vesti- 
bulospinal pathways appear to exist in 
all vertebrates which have been studied 
(Shapovalov, 1972). Thus it may be possible 
to speak of “phylogenetic age” of descend- 
ing pathways. 
(3) While there is a certain conservatism 
regarding the position of particular path- 
ways in the spinal white matter and their 
terminal fields in the spinal gray, individual 
pathways may “shift” even between species. 
These shifts in position and termination 
may be correlated with phylogenetic age of 
the pathway. The phylogenetically new cor- 
ticospinal pathway is found in just about 
every conceivable position in the spinal 
white matter of mammals, including the 
dorsal funiculus which usually contains only 
ascending fibers (Verhaart, 1970) and whose 
terminal fields are equally variable. Such 
shifts in the position and terminal field 
might be related to shifts in the position and 
orientation of the dendritic fields of target 
neurons. 
(4) Even while retaining the same posi- 
tion and terminal field in the spinal cord, 
a pathway may differ by developing more 
collateral branches, more elaborate pre- 
terminal arborizations, and, consequently, 
a greater number of synaptic boutons. Thus 
the rubrospinal pathway, thought to be 
phylogenetically new in reptiles, has, in liz- 
ards, a terminal field less dense, relative to 
other pathways, than in mammals. On the 
other hand, the phylogenetically old reticulo- 
spinal pathways have much more intense 
terminal fields in the contralateral relative 
to the ipsilateral medial spinal gray of liz- 
ards than they do in mammals. The differ- 
ences in these terminal fields may be related, 
functionally, to the particular uses of axial 
rather than limb musculature in lizards and 
thus they may not hold for other reptilian 
species. 
In summary, differences such as the pres- 
ence or absence of a major descending path- 
