DEDIFFERENTIATION IN PEROPHORA 687 
We may now leave the nervous system and return to physi- 
ology. As an example in mammals, and one concerned only 
with the parts of one organ, the following will serve. 
As is common knowledge, the testis in mammals consists of 
several functionally-distinct parts. Apart from blood-vessels 
and nerves there are (1) the germ-cells (spermatogonia, sperma- 
tocytes, spermatids, and spermatozoa), (2) the cells of Sertoli, 
(3) the interstitial cells or cells of Leydig, (4) connective-tissue 
cells. In the normal testis these exist in proportions which 
do not vary beyond narrow limits. Various agencies, however, 
will upset this balance. The germ-cells are the most suscep- 
tible. Exposure of the testis region to X-rays or to Meso- 
thorium; or ligature or section of the vas deferens Ch Or 
abnormal position in the organism, which can come about 
spontaneously as in natural cryptorchism or can be produced 
experimentally as in artificial eryptorchism or by transplanta- 
tion, will bring about some degree of degeneration of the germ- 
cells. This is accompanied in every case by a hypertrophy 
of the interstitial cells. The cells of Sertoli are usually 
unaffected. It would appear that these latter are not cells 
capable of rapid multiplication. The chief competition is 
therefore between the germ-cells and the interstitial cells. 
The former are in some way dominant ; when they are damaged, 
a check on the latter is removed, and their active increase 
results. Whereas removal of the testis to an abnormal environ- 
ment usually results in the permanent disappearance of the 
germ-cells, X-ray treatment, if not very intense, only damages 
them temporarily. Later they regenerate, and finally come 
to have their old proportion once more. The increase in the 
number of interstitial cells only lasts until this regeneration 
starts, and is followed by a decrease. Finally, the normal 
equilibrium is re-attained.1 
' See also R. Goldschmidt, ‘ Biol. Centralbl.’, 36, 1916, p- 160. In 
Lepidopteran testes cultivated in tissue-culture, normal spermatogenesis 
occurs. But the germ-cells always die before the cells of the follicle. 
When this happens, the follicle-cells, which have till then remained 
normal, start at once to multiply at a rapid rate, 
ZZ2 
