THE NERVOUS SYSTEM 



553 



which form the neurolemma sheaths migrate into the nerve anlagen. The 

 medullary sheaths are secreted later by the nerve fibers. The Hensen 

 hypothesis has recently been supported by J. Graham Kerr in his text-book 

 of vertebrate embryology. 



In support of the Hensen hypothesis, Kerr advances evidence derived 

 from the study of fish embryos. The first connexions between myotome 

 and neural tube which he is able to discover are fine plasmodesms which, 

 without evidence, Kerr assumes to be primary. Later, these connexions 

 increase in length and diameter, become fibrillar, and finally are invaded 

 by the mesenchyma cells which form the neurolemma. The elongation 



PLASMODESMS 



EURAL TUBE 



NERVE ANLAGE 



NOTOCHORD 



SCLEROTOME 

 -MYPOCHORDA 



1MEURAL CREST 



NEURAL TUBE 



MYOTOME-^jj^WI' 

 NEUROBLAST S WTI" vji^ 



NERVE ANLAGE^ ' " 



NEURITES-^^\x y^x^^^^ >; 



CHORDA ■ ■ 



SCLEROTOME 

 HYPOCHORDA- 



A.PLASMODESM THEORY B. CELL-CHAIN THEORY. C. PROCESS THEORY 



Fig. 456. — Diagrams illustrating the three chief theories of neurogenesis, based 

 upon sections of elasmobranch embryos in the trunk region. According to the plasmo- 

 desm theory of Hensen (.4) motor nerves are bundles of primary plasmodesmatous 

 connexions between nerve and muscle which have become enlarged through functional 

 activity. According to Balfour (5) nerves arise from chains of cells. The process 

 theory of Kupffer (C) holds that neurites are processes of neuroblast cells. The evi- 

 dence, experimental and other, supports the Kupffer theory. 



of nerve fibers, which has been cited by others as evidence in favor of the 

 outgrowth theory of nerve development, Kerr interprets as evidence of 

 stretching of the primary connexion. In favor of the theory of primary 

 connexion of nerve and muscle, Kerr calls attention to the fact that, from 

 the time of their first appearance in the embryo, neural tube and myotome 

 are in close proximity to one another and hence might easily be connected 

 by plasmodesms. As evidence in favor of the Hensen hypothesis, Kerr 

 mentions the results of an experiment of Spemann who transplanted the 

 limb bud of a frog to its cheek. The leg muscles became innervated by 

 nerves from the cheek. It might appear that such an experiment proves 

 that nerve and muscle may become secondarily interconnected, but Kerr 

 interprets the evidence as favoring the Hensen hypothesis ! 



Of the presence of plasmodesms in vertebrate embryos, there can be 

 not the slightest doubt. The fact that they are more conspicuous in 

 some embryos than in others, and that they are exaggerated by some 



