[118] 



OVUM. 



Annelida. According to this description, the 

 first germs of the ova consist of minute ger- 

 minal vesicles formed in the ovarian sub- 

 stance; they soon acquire the single macula 

 or nucleus. After undergoing some enlarge- 

 ment, these germs fall into the abdominal 

 cavity, and there acquire, by deposit round 

 them, the clear primitive vitelline substance. 

 In this substance opaque granules, which are 

 at first colourless, are subsequently deposited; 

 and as these extend outwards from the ger- 

 minal vesicle, and accumulate in quantity so 

 as to increase the bulk of the whole ovum, a 

 delicate vitelline membrane is added exter- 

 nally. The germinal vesicle attains a diameter 

 of about -$\v", and its macula of ^-^3"; and 

 when the several parts of the ovum which 

 have been mentioned have appeared, and the 

 yolk is now coloured, the whole ovum has a 

 diameter of about 75-^". 



The superficial part of the yolk consists of 

 minute coloured granules. Within this there 

 are larger oil-like globules free of colour, and 

 in the innermost part a somewhat viscous 

 transparent fluid.* 



According to Leydig, the germinal vesicle 

 in Piscicola becomes enveloped by a second 

 vesicle or cell-wall before the formation of 

 the yolk-substance ; but it is suggested by 

 Leuckart that he may have been misled in 

 this by the appearance often presented by 

 the clear and somewhat highly refracting 

 substance which in many animals precedes 

 the formation of the opaque yolk. If" this is 

 not so, the fact observed by Leydig would 

 constitute a marked departure from the usual 

 homological relations of the ovum.f 



Rotifera. Although most zoologists are 

 now disposed, on the ground of the analogies 

 in the most important parts of their general 

 structure, to place the Rotifera among or close 

 to the Articulate Worms, yet in some re- 

 spects their mode of reproduction presents a 

 marked correspondence with that of the 

 lower Crustacea. Thus they have, in com- 

 mon with some of the lower Crustacea, the 

 occasional separate condition of the sexes, 

 the preponderance of females, the imperfect 

 development of the males, the proportionally 

 large size of the ova, and the production of 

 winter ova as well as the ordinary kind ; 

 on the other hand, the simpler structure 

 of the ovum and its complete segmenta- 

 tion are more similar to what is observed 

 among the Vermes.f 



* Quatrefages, it is to be observed, designates 

 the enveloping membrane ovarian and not vitel- 

 line membrane, which last he holds is wanting in 

 these ova. 



f Farther interesting views of the ova of this 

 class will be found in Milne Edward's memoir in 

 theAnnal.des Scien. Nat. for 1845, vol. xxiii. p. 145 ; 

 and in his article ANNELIDA in this Cyclopaedia, 

 to which I must refer the reader ; in Grube's Unter- 

 such, iiber die Eutwickel. der Clepsine, Konigsberg, 

 1844. H. Koch, Ein Worte zur Entwick. von 

 Eunice, with an Appendix by Kolliker, on Exogone 

 and Cystonereis. 



| See Leydig, On the Structure and Systematic 

 Position of the Rotifera, &c., in Zeitsch, fur Wissen. 



The relation of the sexes in Rotifera 

 has only recently been in any degree under- 

 stood, and that only in a few genera ; and 

 there are still many points requiring elucida- 

 tion. The greater number of the animals, in 

 fact, which till lately have been known or de- 

 scribed in this class have been females ; and as 

 yet the males or male organs have been as- 

 certained only in a few genera. Some are 

 certainly of separate sexes, as Notommata, 

 and the allied Rotifer of which the male 

 was first discovered by Brightwell*, and of 

 which the development was described by 

 Dalrymplef, Others seem to be hermaphro- 

 dite, as in Megalotrocha, described by Kol- 

 liker^; ; in Euchlanis, by Schmidt^ ; and in 

 Lacinularia socialis, by Leydig. || But ac- 

 cording to Huxley If, there may still be 

 some doubts as to the bodies described as 

 spermatozoa, and as to the arrangement of the 

 male organs in the Lacinularia. 



Fig. 83*. 



Ovarian ova of Rotifera. (From Huxley,') 



The figures represent the formation and develop- 

 ment of the true or ovarian ova of Lacinularia 

 socialis (one of the Rotifera). A. and B. are small 

 fragments of the ovarian substance showing the 

 primitive ova with their germinal vesicles and 

 maculae ; in B. one of the ova more advanced than 

 the rest. c. represents the mature ovum. D. the 

 same undergoing the first stage of segmentation. 



The ova of Rotifera have been observed 

 by Ehrenberg and many other microscopists. 

 They are of comparatively large size, but yet 

 belong to the group of ova possessing the 

 simpler kind of structure, the yolk substance 

 being quite finely granular, and undergoing a 

 complete segmentation. The germinal vesicle 

 is large, and possesses a distinct single ma- 

 cula ; and the whole ovum is inclosed in a 

 clear vitelline membrane. No micropyle has 

 yet been discovered, nor have the time and 



Zool., vol. vi. ; and C. Vogt on the same subject 

 in vol. vii. of the same work. 



* Ann. of Nat. Hist, for Sept. 1848, p. 153. 



t Philos. Transact, 1849, p. 331. 



t Froriep's Neue Notizen, 1843, p. 17. 



Vergleich. Anat. p. 268. 



|| Zeitsch. fur Wissen. Zool., vol. iii. 



^f Microscop. Soc. Trans, p. 1. in vol. i. of Microsc. 

 Journal, 1853. 



