1892.] Heredity and the Germ-Cells. 649 
widely differentiated in form, the ovum large and passive, the 
spermatozoon small and active. The readiest induction was 
to regard these elements as representing distinct physiological 
principles, corresponding to the essential sexual characteris- 
tics—in short, as male and female cells, the former vitalizing 
and rejuvenating the latter. Thus one of the earliest definite 
“ polar-body ” theories was that the ovum was hermaphrodite, 
containing both male and female principles, and that it was 
necessary to get rid of the male substance before the spermat- 
ozoon could enter. 
As Von Siebold and Leuckart had demonstrated that some 
ova reproduce parthenogenetically, that is without fertilization 
by spermatozoa, Weismann turned to such forms for the solu- 
tion of this problem, and was surprised to find that partheno- 
getic ova only extrude one polar body; this led him to attach 
one meaning to the first polar body and another meaning to 
the second, which he viewed as designed to reduce the heredi- 
tary substance in the ovum without regard to sex. Thus both 
this and the older theory conveyed alike the idea of reduction, 
but with an entirely different supposition as to the nature of 
the material reduced or eliminated. 
Maupas on Conjugation among the Infusoria.—Among the 
newer researches which throw light upon this old problem 
those of Maupas are certainly the most brilliant. After a most 
exact and arduous research, extending over several years, he 
collected his results in two memoirs, published in 1889 and 
1890. 
His experiments were first directed upon the laws of direct 
multiplication by fission, which revealed a complete cycle of 
life in the single-celled Infusoria and showed that after a long 
period this mode of reproduction becomes less vigorous, then 
declines, and finally ceases altogether unless the stock is reju- 
venated by conjugation of individuals from different broods. 
In other words, these broods of minute organisms grow old 
and die unless they are enabled to fertilize each other by an 
‘Sur la multiplication des Infusoires Ciliés, Archiv de Zoologie expérimentale, Sér. 
3, vol. vi, pp. 165-278; Le Rejeunissement Karyogamique chez les Ciliés, vol. vii, 
pp- 149-517. See also Hartog, Quart. Journ. Microscop. Science, December, 1891. 
