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. 4S5.-A 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 f unicuH. 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 hypothesis is based upon the assumption that an embryo 

 is not a mosaic of cells but a syncytium or mass of protoplasm containing 

 numerous nuclei but without cell boundaries. Hensen asserts that the 

 protoplasm of a vertebrate embryo is continuous and unbroken; that 

 when cells divide, the division is never quite complete, but connexion is 

 retained by means of protoplasmic threads or plasmodesms. Not all of 

 these plasmodesms, however, become nervous. Some degenerate and 

 disappear during ontogenesis. The plasmodesms which later become 

 nerves do so as a result of nervous activity and consequent enlargement. 

 Within them eventually appear neurofibrillae. The mesenchyma cells 



