Section VII 

 SPECIAL VERTEBRATE ORGANOGENESIS 



CHAPTER 1 



Nervous System 



(Neurogenesis) 

 PAUL WEISS 



THE OBJECT AND THE PROBLEMS 



We are to deal here with the causal analysis 

 of "the development of the nervous system." 

 In that generality, the task is simply un- 

 manageable. Of the innumerable aspects the 

 mature nervous system offers to the observer, 

 each one has had its characteristic ontoge- 

 netic history, hence raises separate questions 

 as to time and manner of its origin and as 

 to mode and means of its ontogenetic trans- 

 formations. This points us to the only prac- 

 tical approach, which lies in resolving the 

 confusing complexity of the system into 

 simpler components and addressing our 

 questions to the more elementary events thus 

 singled out. Most of the following account 

 will be essentially a sample exercise in 

 phrasing and sorting such questions of suffi- 

 cient concreteness as to offer hope for precise 

 answers. The answers themselves are mostly 

 still in a very fragmentary state and will 

 be presented without glossing over their 

 often provisional character. I have chosen 

 topics and examples chiefly in the spirit of the 

 guiding theme of this book, which is to il- 

 luminate, rather than cover, the processes of 

 development. This also explains the argu- 

 mentative, rather than reportorial, manner 

 of presentation. It reflects the effort to give 

 a coherent and consistent pictm-e, in which 

 facts and data are rated not as isolated items, 

 but as tools for the clarification and solution 

 of problems — not as sheer statements, but as 

 answers to questions; which makes the text 

 useful as a guide more to the understanding, 

 than to the literature, of the field. 



Some familiarity with the main morpho- 

 logical, physiological, and embryological 

 featm-es of the nervous system will be taken 

 for granted. Yet a brief listing of the most 

 prominent ones may help to keep our 

 analytical questions properly focussed from 

 the start. Somewhat arbitrarily we shall 

 separate the discussion of the central nervous 

 system, which serves intercommvinication 

 among its constituent units, from that of 

 the peripheral nerves, which serve communi- 

 cation between the former and the non- 

 nervous tissues of the body. 



THE PERIPHERAL NERVE 



Nerves are composite structures, contain- 

 ing bundles of nerve fibers of different classes, 

 associated in variable numbers, proportions, 

 and groupings, and held together and 

 sheathed by connective tissue, in which 

 course blood and lymph vessels and endo- 

 neurial fluid. In the so-called plexuses, nerves 

 regroup or exchange some of their fibers. 

 Farther peripherally, they branch by succes- 

 sive dichotomies and distribute their branches 

 over the periphery according to patterns 

 characteristic of the given peripheral sector 

 or organ, with considerable latitude for 

 individual variation. 



The component nerve fibers themselves are 

 composite (Fig. 125), with the axis cylinder 

 (axon or neurite, a)— a protoplasmic exten- 

 sion of the centrally located cell body (peri- 

 karyon) — at the core; covered by a membrane 

 or medullated sheath {m) consisting of alter- 



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