452 GENERAL HISTORY OF THE INFUSORIA. 



require fecundation, and are thence to be considered as a mode of asexual 

 reproduction." 



The multicellular character of the contents of these ^ ephippial ' ova, Cohn 

 is unable to confii'm. In his very valuable essay on the '* Development of 

 Rotatoria " (Zeitschr. 1855), this able observer has promulgated the hj^jothesis 

 of the occurrence of the phenomenon of " alternation of generations," of par- 

 thenogenesis or virgin- development. A resume of the reasons for this view 

 may stand thus : — Female Rotifera lay eggs of only one sex ; and winter eggs 

 are produced only by certain females and at certain periods — contemporane- 

 ously, that is, Avith the generation of males : again, the males are too few to 

 impregnate the whole of the apparent female beings, which are so largely 

 found, and always replete with ova in course of development, at all seasons. 

 The conclusion, therefore, forces itself upon us that the common ''summer" 

 ova are produced Avithin the parent animal without any antecedent genera- 

 tive act or impregnation ; that is, in other words, they are asexual products or 

 germs. If this be tiTie, it follows that the beings producing them are not 

 true females, but merely asexual nurses (Ammen), fiu'nished Tsath a germinal 

 mass, but destitute of a real ovaiy, and not demanding the action of the male 

 for the development of its germinal elements. On the other hand, the 

 " winter " must be considered the tnie ova, and the beings producing them 

 the only true females, furnished with an ovary, to which the energy of 

 the speiTnatozoa of the male is necessary. But, notA\athstanding these phy- 

 siological differences, the mere niu'ses and the actual female Rotifera are in- 

 distinguishable in stinicture. In illustration of this hypothesis, its analogy 

 with what occurs in Aphis, Daphnia, and Artemia, may be quoted. 



Of the rate of development of these winter eggs, we know little. Huxley's 

 account would render it a final act, involving the sacrifice of a large, or even 

 of the largest, part of the ovary, and consequently one which we cannot sup- 

 pose capable of frequent repetition. Leychg has, indeed, an observation which, 

 if accurate, proves a rapid reproduction of such ova by the ovary. He 

 informs us he observed an isolated individual of Notommata Myrmeleo lay 

 the solitary bristle-shelled winter o\^im which its oviduct contained about 

 12 o'clock in the day ; and on rencAATiig his researches at 3 in the afternoon, 

 discovered another such egg completely formed in the ovaiy. 



This author recounts also, in his histoiy oi Notommata Sieboldii, the following 

 particulars, which, if confinned, would prove the formation, whether of winter 

 or of summer ova, to be determinable by accidental external cii'cumstances : 

 — " When I kept the Notommata for some days in clear water containing no 

 nutriment, the ovary shrivelled, the granular mass (yelk) altogether vanished, 

 the germinal vesicles became simple bodies, and all such individuals produced 

 only winter eggs." 



That the Rotatoria, on the approach of winter, are Likely to be placed under 

 conditions in which food is scarce, and which are unfavourable to vigorous 

 life, is at once admissible, and, if Ley dig's observation be correct, furnishes 

 an explanation of the generally apparent limitation of the production of 

 winter ova to that season. Be this as it may, the -winter ova must be re- 

 garded as indicating the conservative tendency of natiu-e in providing for the 

 continuance of the species by organisms so constmcted as to endui^e the 

 severity of the winter season, and to retain a dormant vitality through it, 

 imtil the genial influence of spring awakens them into activity and hfe. 



Fecundity of Rotatoria. — Although the Rotatoria are not endowed with 

 the various faculties of reproduction possessed by the Protozoa, yet their vast 

 increase by eggs only is astonishing. Ehi'enberg wrote that he insulated 

 a single specimen of HyrJatina senta, and kept it in a separate vessel for 



