1898] REPRODUCTION OF THE ROTIFER A 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-eggs throughout life. This 

 delimitation of the conditions necessary for successful fertilisation 

 explained the older observations of spermatozoa within females 

 laving 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-eggs 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 werneclcii 

 [Iiothert]), 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 (33) 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 impossible 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 



