DISTURBANCES OF NEURAL FUNCTION IN THE PRESENCE OF CONGENITAL DISORDERS 



I9'7 



which expresses itself over go per cent of the time. 

 When the tumor has not killed the individual until 

 after adulthood is reached, reproduction of course 

 becomes possible. From the families of such persons, 

 the genetic data are known (25). 



The neurophysiology disturbances from the tumor 

 are those referable to local destruction of the eye by 

 the tumor, and to invasion of the brain or adjacent 

 nerves. There are of course first focal defects in the 

 retinal field followed usually by loss of vision in that 

 eye. Other alterations of neural function would vary 

 with the course of the invasive tumor: compression 

 and destruction of the nerves of the orbit, destruction 

 and compression of the anterior parts of the brain, 

 and increased intracranial pressure. 



The example is chosen to illustrate still another 

 way in which neurogenesis may go wrong. In this 

 instance a genetic situation expresses itself as a de- 

 structive neoplasm, perhaps first passing through a 

 stage of nonneoplastic structural malformation. 



EXPERIMENTAL CONGENITAL ANOMALIES 



In a sense, any form of abnormal development 

 brought about by experimental embryologic extirpa- 

 tive techniques or other procedures is a congenital 

 anomaly. In many instances tin- difference between 

 spontaneous and induced structural abnormalities is 

 simply a matter of whether the procedure was the 

 result of an 'accident' or designed by an experimenter. 

 There are two areas of experimental neuroembryology 

 that might not ordinarily come into a discussion of 

 congenital anomalies but which deserve mention be- 

 cause they illustrate some most interesting aspects 

 of abnormal neural functional development. Thev 

 arc a) the effects of heteroploidy on the ontogen) of 

 the brain and b) the degree of functional specificity 

 that is built into the structure of the nervous system 

 early in its development. This second, however, is so 

 much a part of experimental neuroembryology and 

 'normal' developmental neurophysiology that it will 

 not be reviewed here. Reference has been made to 

 the work of Weiss, Sperry and Stone in the brief 

 summary of neural development. Suffice it to s.iv 

 that the work emphasizes that what we call 'abnormal' 

 development is as much governed by ontogenetic 

 laws as the normal. 



Heteroploidy in Salamanders 



The phenomenon of how ploidy in development 

 can affect the function of the nervous system is suffi- 



ciently interesting and unique to justify its separation 

 from other "structural" disorders. In the vertebrates 

 generally it is usually taken for granted that the nor- 

 mal complement of chromosomes in somatic cells is 

 diploid, that is twice the (haploid) number in the 

 sperm or egg. This is certainly approximately true, 

 although in adult mammalian cells variations from 

 cell to cell are known to occur and variation in some 

 degree may be the rule. Parthenogenesis, too, occurs 

 not only in lower vertebrates but in birds and mam- 

 mals including, probably, man (in the latter, however, 

 parthenogenetic embryonic development would not 

 be credited if it progressed beyond the earliest stages). 

 In lower vertebrates animals may develop with not 

 only the normal two sets of chromosomes but with 

 three sets (triploidy), four sets (tetraploidy) and even 

 up to seven sets. Fankhauser and his associates have 

 studied the general consequences of heteroploidy in 

 development (13) and particular aspects of neural 

 function in such abnormal animals (14). 



Polyploid animals may occur spontaneously among 

 salamanders (Triturus viridescens), and animals with 

 three sets of chromosomes can be induced by exposing 

 fertilized eggs to a temperature of 36°C which sup- 

 presses the second maturation division. What is of 

 interest here in polyploid salamanders is that (In- 

 individual bod) cells are larger than normal, but the 

 whole animal is only normal size. As .1 result (here is 

 actually a smaller total number of cells in the body, 

 perhaps something like two-thirds the normal num- 

 ber. Cross-sections show this to be true in the triploid 

 brain which is normal size but obviously has less 

 cells. (With increasing numbers of sets of chromo- 

 somes, there are proportionately even fewer cells, 

 although the discrepancy does not adv. nice on 

 straight numerical scale. 



Fankhauser and his co-workers compared the 

 learning capacity of triploid salamanders with that 

 of normal diploid animals. They were subjected to a 

 swimming course involving a choice between a reward 

 and a noxious stimulus, and their ability to differen- 

 tiate between the two was tested. The triploid animals 

 were less gifted than their brothers in this respect, 

 and the evidence gathered pointed to a lesser number 

 of brain cells as being the principal factor responsible 

 for their deficiencies. 



Whether heteroploidy is ever a significant factor in 

 the development of abnormal function of the nervous 

 system in higher vertebrates is presently unknown. It 

 serves however as still another example of how de- 

 velopmental processes can express themselves in 

 terms of deviations from normal neural function. 



