THE ECTODERM AND ITS DERIVATIVES 



169 



anterior and posteriorly directed fibers of the various neurons are con- 

 centrated more laterally so that only cross sections of axons will be 

 seen. This peripheral area is then known as the white matter, to dis- 

 tinguish it from the more central and cellular gray matter of the cord. 

 In addition to the lateral walls of the cord, which are thick, the 

 dorsal wall is also thick because it consists of the fused neural folds. 



Dorsal column of white 

 matter 



Ventro-laferal column 

 of wtiite matter 



Neurocoel (spinal canol) 



Gray matter (oxons) 



Ependymol cells 



Dorsal root ganglion 

 (from neurol crest) 



Afferent nerve troct 



Dorsal horn ] .. 



[ Neuro 

 I blasts 



Peripheral nerve 



Spinal artery 



Sympathetic ramus 



Development of the spinal cord of the frog. (Top) 

 Spinal cord of the 7 mm. larva. {Bottom) Spinal cord 

 just before metamorphosis. 



The central canal or neurocoel is therefore displaced ventrally in the 

 embryo and larva, but, as the cord is developed and more neuroblasts 

 are formed, the neurocoel assumes a more central position. Within 

 the enveloping connective tissue membrane which surrounds the spinal 

 cord may be seen several blood vessels, principally the large spinal 

 artery located in the mid-ventral (inverted) groove of the cord. 



The Peripheral Nervous System 

 The development of this variegated portion of the nervous system 



will be treated in the following sequence, and the description will be 



continuous to the final stage of development: 



1. Organs of special sense: optic, auditory, olfactory, and lateral 



line organs. 



