. Comparative anatomy. Anatomy, Comparative. 552 COMPARATIVE ANATOMY and more numerous processes, and which do not extend through the entire thickness of the wall. Some of the spongioblasts form transient neurolemma cells which enclose the neurites while the myelin sheaths are formed. Most of these dis- appear in the adult cord. Some of the ependymal cells persist as the epithelial lining of the lumen of the cord. The marginal layer gradually thickens by the addition of libers, some of which grow craniad and some caudad, and most of which soon acquire a myelin sheath. "fig. diagram
. Comparative anatomy. Anatomy, Comparative. 552 COMPARATIVE ANATOMY and more numerous processes, and which do not extend through the entire thickness of the wall. Some of the spongioblasts form transient neurolemma cells which enclose the neurites while the myelin sheaths are formed. Most of these dis- appear in the adult cord. Some of the ependymal cells persist as the epithelial lining of the lumen of the cord. The marginal layer gradually thickens by the addition of libers, some of which grow craniad and some caudad, and most of which soon acquire a myelin sheath. "fig. diagram illustrating the ^ith the appearance of dorsal and theory of neurobiotaxis of Ariens-Kappers. Ventral nerve roots in Connexion The diagram represents a series of motor ^j^j^ ^^^ j^^j ^^^^ ^-^^ marginal neuroblast cells lying m the wall of the _^ ' ° spinal cord as activated to form neurites zone of white fibers bccomes divided by a longitudinal bundle of neurites As ^^^^ ^^^g^j lateral, and Ventral the bundle of neurites passes a neuroblast, ... a stimulofugal process is formed. Later by funicuH. Since the time of myelin- a reverse or stimulopetal process the ^^^^^ q£ ^^ers differs in the several neuroblast is drawn towards the activating bundle and dendritic processes grow towards tracts, depending upon the time the bundle. Successive stages in the proc- .^^^en they begin tO function, some, ess are shown beginning at the top of the' . diagram. (Redrawn after Ariens-Kappers.) like the pyramidal, become medul- lated only after birth. DEVELOPMENT OF PERIPHERAL NERVES The problem of how nerves become connected with their end-organs in skin, muscle, or gland has been a controversial issue down to the present. There is, however, no doubt that all nervous tissues are of ectodermal origin. Three theories of neurogenesis may be mentioned. The Hensen Theory. According to the Hensen theory, which has been revived in this century, the connexion between nerve and muscle is primary. This
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