1898] REPRODUCTION OF THE ROTIFERA 47 
only if fertilisation took place at a very early age before they had 
begun to lay eggs. On older females or on those capable of pro- 
ducing female eggs, impregnation, if it took place, was without 
result. When fertilisation was successfully accomplished, however, 
the individual produced only resting-eges throughout life. This 
delimitation of the conditions necessary for successful fertilisation 
explained the older observations of spermatozoa within females 
laying other than.resting-eggs, as well as Plate’s failure to trace the 
history of the spermatozoa within the body of the female. The 
evidence as to the fertilisation of the resting-eggs has been completed 
by the observations of Sadones (31), who observed the passage of 
spermatozoa through the ovarian wall in Hydatina, and of Lauterborn 
and v. Erlanger (32), who have traced the fusion of the male and 
female pronuclei in the resting-eges of Asplanchna. Nussbaum (30) 
was unable to satisfy himself of the correctness of Maupas’ view 
that fertilisation only affected eggs which would otherwise have 
given rise to males, but confirmatory evidence on this point is 
afforded by Lauterborn’s observations on <Asplanchna. In this 
viviparous form, Masius (18) had already noted the occurrence of a 
resting-egg in the oviduct along with ordinary parthenogenetic eggs 
or embryos, and Lauterborn, confirming this, finds the latter to be 
always males. The fact that in Asplanchna the same individual 
may produce both ‘ resting’ and parthenogenetic eggs (as is perhaps 
also the case with Synchaeta [Apstein] and Notommata wernecki 
[Rothert]), indicates that in this respect Maupas’ results as to 
Hydatina are not to be extended without qualification to other 
forms. The rule that male and female eggs are produced by different 
individuals may also be not without exceptions, especially in the 
genus Brachionus, where Daday (fide Weber, 15) and Wesenberg- 
Lund (83) have observed both kinds of eggs carried on the carapace 
of the same individual. In this connection it may be remarked that 
the difference in size between the two sexes, and therefore between 
the eggs producing them, is not nearly so marked in Hydatina, for 
instance, as in such forms as Brachionus ; and that in the former genus 
Nussbaum and others have observed a continuous gradation of sizes 
between the two, so that in some cases it was inpossible to determine 
the sex of an egg without observing the development of the contained 
embryo. It is possible that want of sufficient attention to this point 
may have led to some of the discrepant results mentioned above. 
Of course, when the embryos have reached a certain stage, the 
presence or absence of the mastax at once indicates their sex. 
The question of the causes which lead to the appearance of 
males and the consequent production of resting-eggs is one of great 
interest, and has received several answers, none of which can yet 
be regarded as quite satisfactory. It has long been known that the 
