Nervous System 



389 



still incomplete nervous systems; although 

 in view of oiu- concept of the continuous 

 growth of neurons (see p. 364) and the in- 

 stability of their connections (see p. 363), 

 particularly in the submicroscopic realm, 

 the distinction between the two groups is 

 apt to fade. The former group is best illus- 

 trated by the hormonal effects on sexual 

 behavior (see Beach, '47), the latter by the 

 hormone dependence of the transformation 

 of neural structures and functions from the 

 larval to the adult state in metamorphosing 

 species. 



thyroxine carrier) near the abducens nu- 

 cleus, which effects the central linkage 

 (Kollros, '43a). Similarly, the morphologi- 

 cal signs of brain metamorphosis, such as 

 increased proliferation, cell increase and 

 histogenetic changes, can be locally evoked 

 (Kollros et al., '50; Weiss and Rossetti, '51). 

 Particularly instructive in the latter sense 

 is the response of Mauthner's neuron, which 

 normally regresses during metamorphosis; 

 if metamorphosis of the hindbrain is en- 

 forced by the local application of a thy- 

 roxine source, the nerve cell bodies enlarge 



I 23456789 10 



DIAMETER CLASSES 



Fig. 143. Metamorphic size changes of opposite sign in Mauthner's cells (M-cells) and their surrounding 

 neuron population (non-M-cells) in urodele larvae with thyroid or thyroxine-agar implants near brain. 

 Histograms of nuclear diameters in 1000 non-M-cells each of control, thyroid and thyroxine cases, as well 

 as mean diameters of M-cells for the corresponding groups of animals. (From Weiss and Rossetti, '51.) 



As outlined in Section XII, amphibian 

 metamorphosis is under the direct control of 

 the thyroid hormone; insect metamorphosis 

 similarly depends upon the hormones of cer- 

 tain head glands. The directness of the con- 

 trol is proven by the fact that upon localized 

 topical application of hormone, only the sur- 

 rounding local region of the tissue undergoes 

 transformation. The same has now been dem- 

 onstrated for the "adaptive" changes in the 

 nervous control mechanism which must 

 necessarily accompany the other bodily 

 transformations if the metamorphosed ani- 

 mal is to function properly. That these 

 neural changes are likewise under the pri- 

 mary control of the hormone is revealed by 

 the following experiment. The "wink" re- 

 flex of the frog, which is normally not ex- 

 ecuted by the larva although sensory and 

 motor elements are present and individually 

 capable of functioning (Kollros, '42), can 

 be made to mature precociously by the im- 

 plantation of thyroid gland (or an artificial 



markedly (Fig. 143) save for Mauthner's 

 cell, which shrinks (Weiss and Rossetti, '51). 

 This result proves that the various neuron 

 types respond to the hormone each in its own 

 distinctive fashion, determined by preformed 

 metabolic patterns, which further substanti- 

 ates the qvialitative diversity within the neu- 

 ronal population emphasized throughout our 

 discussion. 



Again these experiments are only modest 

 openings into a rich field of future investi- 

 gation, for in no other event of neurogenesis 

 do we find such a favorable constellation of 

 circumstances as in metamorphosis, where 

 dramatic changes in neural composition, 

 architecture and function, crowded into a 

 relatively brief span of time, supervene in 

 a nervous system that had already attained 

 an advanced state of functional perfection, 

 whose parts are distinct, relatively large and 

 easily accessible to manipulation, and whose 

 transformations can be set off by a con- 

 trollable agent. 



