DEVELOPMENT OF REFLEX MECHANISMS i) 
fibers, or they may come from the brain through the various 
cerebro-spinal tracts—from the visual centers through the trac- 
tus tecto-spinalis, from the auditory or vestibular centers through 
the tractus bulbo-spinalis, ete. Furthermore, the long ascend- 
ing systems between the spinal cord and the brain (spino-bulbar, 
spino-cerebellar and spino-tectal tracts) may also discharge 
collaterals among dendrites of these peripheral motor neurones. 
The Amblystoma larva is capable of making only the simplest 
responses by means of the body musculature, of which the chief 
is the simple swimming movement; but, unlike the young embryos 
considered in the preceding section, these responses may be called 
forth by a great variety of means. The body musculature here 
is responsive to environmental influences received through many 
more avenues of sense, and its movements are, in addition, under 
the control of the higher correlation centers of the brain. 
THE MAMMALIAN SPINAL CORD 
The mechanism of mammalian spinal reflexes need not be here 
reviewed, especially since our present knowledge of the subject 
has been so admirably summarized by Sherrington (06). The 
most significant feature here from our present standpoint is the 
further advance in the differentiation of the effector apparatus. 
Though the spinal cord of larval Amblystoma may be excited 
by means of a wide range of receptors, the typical response is a 
total reaction of essentially the same sort in all cases. In mam- 
mals the differentiation of the limbs and other highly special 
motor organs permits of an extensive series of refined movements; 
and in conformity with this diversity of response we find the cor- 
relation neurones of the spinal cord organized into elaborate 
functional systems, each of which may act either independently 
or conjointly with others upon particular groups of synergic 
motor neurones. The cord of larval Amblystoma may be con- 
ceived as a house with many entrances and but one exit; that of 
a mammal with still more entrances and also a large number 
of exits. 
