538 Transactions of the Society. 



of development in the same individual. The vitellus, at first homo- 

 geneous and colourless, encloses a clear germinal vesicle, having 

 itself a bulky Wagner's spot. Later on it becomes granular and 

 yellowish, the germinal vesicle forming a clear rounded spot, and 

 looking like a solid brilliant nucleus when treated with acetic acid. 

 As the eggs approach maturity they are piled one on another in the 

 ovary, mutually flattening each other. The distension they cause 

 ends probably by rupturing the ovarian sac, for later on they are 

 found free in the cavity of the body, immediately beneath the external 

 integument. The animal then presents the rounded form of 

 Fig. 9, with two short prolongations, one at the head, the other at 

 the tail. 



Like many other rotifers, N. WernecMi produces summer eggs 

 and winter eggs, the former destined to be immediately hatched, the 

 latter passing through the cold season, and only hatching the fol- 

 lowing year. They diff'er from each other from the moment they 

 are laid"^ the summer eggs being smaller than the winter eggs 

 (the former being 0-056 mm. by 0*042 mm., and the latter 

 • 062 mm. by • 050 mm.). They present other difierences in their 

 contents, and the number and character of their envelopes. The 

 summer egg has a clear, transparent vitellus, uniformly granular ; 

 a thin chorion constitutes the sole envelope (Fig. 11). In the 

 winter egg, the vitellus is brown and opaque ; it has a diffused, 

 clear, rounded spot in its centre, probably due to the persistence 

 of the germinal vesicle (Fig. 13). There are two envelopes sepa- 

 rated from one another by a space filled with a clear liquid ; the 

 external one thicker {ch), forming a somewhat solid shell, and the 

 internal one thin and membranous, closely apphed to the vitellus {cli). 



Since Dalrymple's * discovery of the existence of small male 

 individuals of N. {Asplanclma) Anglica, two opinions, diametrically 

 opposed to each other, have existed among scientific men on the 

 mode of development of summer and winter eggs. Some maintain 

 with Huxley that the summer eggs alone require for their develop- 

 ment the male element, whilst the winter eggs are produced without 

 previous fecundation. Cohn t and others think, on the contrary, 

 that the latter require sexual impregnation, whilst the former 

 develop by virtue of an intrinsic fecundity (parthenogenesis). Cohn 

 supports this opinion by the fact that in all the species where 

 winter eggs are found, their appearance always coincides with that 

 of the males, as in many Hydatinnea and Brachionaea ; whilst in 

 the whole family of Philodineea, where these eggs are unknown, no 

 males have been observed. 



* " Description of an Infusory Animalcule allied to the genus Notommata of 

 Ehrenberg," 'Phil. Trans.,' 1849. 



t "Ueber die Fortpflauzung der Radertliiere," 'Zeitschr. wiss. Zool.,' 1856, 

 vol. vii. p. 4S3; IS.'iS, vol. xi. p. 293. 



