File:Analysis of development (1955) (18143868836).jpg

Title: Analysis of development
Identifier: analysisofdevelo00will (find matches)
Year: 1955 (1950s)
Authors: Willier, Benjamin H. (Benjamin Harrison), b. 1890
Subjects: Embryology; Embryology
Publisher: Philadelphia, Saunders
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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Nervous System 385 the complete absence of sensory innervation (Weiss, '41a), it must have occurred by way of the motor axon itself. While the earlier experiments were car- ried out in functional larval stages, requiring the remodulation of neurons that had already been modulated once before, the results are the same after embryonic transplantations (Detwiler, '25b, '42). On the other hand, modulation has thus far been proven experi- mentally only in larval amphibians. After the locality of origin of the transplanted patch, rather than bearing the "local sign" (Miner, '51). Similar results have been de- scribed for vestibular neurons (Sperry, '45). The retina likewise consists of a mosaic of sectors of different constitutional specifici- ties which are projected into the optic nerve fibers, thereby enabling the latter to establish selective discharge relations with a corre- sponding central mosaic of specific receptor units in the midbrain roof (Sperry, '43,
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Fig. 142. Myotypic function of supernumerary muscles. A transplanted limb with reversed symmetry (right limb) near a normal left limb mirrors the movements of the latter. (From motion picture, Weiss, '52a.) metamorphosis, neurons lose their plasticity and retain the specificity acquired previ- ously. Rat nerves transposed to other muscles postnatally likewise failed to undergo re- modulation (Sperry, '41). At the same time, there is strong evidence that modulation takes place in prenatal stages in all mammals, including man (Weiss, '35). Sensory neurons are subject to the same qualitative modulation by their respective end-organs as are motoneurons. Propriocep- tive fibers connected with any kind of mus- cle signal to the cord the correct name of the particular muscle (Verzar and Weiss, '30). Trigeminal neurons of the skin newly made to innervate transplanted cornea thereby ac- quire corneal character and corresponding reflex relations (Weiss, '42; Kollros, '43b). A transplanted larval skin patch from a for- eign sector imparts its foreign specificity to the local cutaneous fibers that innervate it; thereafter when these fibers are stimu- lated they evoke reflexes characteristic of '44). In urodele embryos this qualitative mo- saic condition is attained about the neurula stage (Stone, '44). While the rather general validity of the principle of specific neuron modulation by effector and receptor organs seems thus well established, the nature of the processes in- volved is still undefined. To judge from its slow rate, as well as its qualitative diversity, modulation belongs to quite a different class of processes than nerve conduction. Its speci- ficity sets it apart from more general "trophic" effects. Perhaps it resembles most closely phenomena of induction, infection and immunological sensitization. Modulation, it must be stressed, clarifies only one aspect of what really is a two- sided phenomenon. By labelling, as it were, the ganglion cells according to their ter- minations, it produces a qualitative point- for-point replica in the centers of the periph- eral receptor and effector units. Possibly the "labelled" neurons can, in turn, transfer

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