OVUM. 



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into the cavity behind, in which an increased 

 growth of substance round the ova and within 

 the shell gives rise to the production of a two- 

 valved case for containing the ova. Accord- 

 ing to S. Fisher of St. Petersburg *, the for- 

 mation of the ephippial ova may be noticed 

 during the whole season, from the middle of 

 July onwards ; and it may therefore be inferred 

 that these ova have for their object the pre- 

 servation of the species in the heat of summer 

 when the ponds are liable to be dried up, as 

 well as by resisting the cold of winter. 



Von Sieboldf states that these hibernating 

 ova contain no germinal vesicle ; and Dr. 

 Burnett, in his translation of Von Siebold's 

 work, has adduced various arguments in fa- 

 vour of the view that this is an instance of 

 "internal gemmiparity" (as he regards the 

 corresponding phenomenon in Aphides) rather 

 than the production of true ova. "Sufficient 

 data are still wanting, however, to form a de- 

 cided opinion on this subject, as we cannot 

 at present distinguish between the ova of the 

 Entomostraca which are the result of fecun- 

 dation, and those which are formed and de- 

 veloped independently of the concurrence of 

 the male.J 



Anmdata. In the class of Annulate Worms, 

 including the Leeches, Earthworms, Nereids, 

 and Amphitrites$ , although considerable va- 

 rieties present themselves in the modes of 

 reproduction, there is yet a greater degree of 

 uniformity in the structure of the ova than 

 in some of the classes previously referred to. 



In the greater number the ova are nearly 

 spherical in form, of rather small size ; the 

 yolk-substance is generally finely granular, 

 and segmentation is complete; the germinal 

 vesicle is clear, with a distinct single macula, 

 or one which is elongated or only slightly 

 divided into subordinate particles. In most 

 ova of Annulata there is, in addition to the 

 inner transparent vitelline membrane, a cho- 

 rion or external membrane of considerable 

 strength, and not unfrequently a superadded 

 layer of albuminous substance, which unites 

 the ova in groups or cocoons, or serves to 

 attach them to other bodies. \ 



In Clepsine, among the Hirudinea, the yolk- 

 substance differs from the common form above 

 described, being composed rather of larger- 

 sized globules ; and in another genus belong- 

 ing to the same order, Piscicola, according to 

 Leydig \, there are peculiarities of structure 



* Mem. of the St. Petersburg Acad., 1848, torn, 

 vi., p. 162. 



f Compar. Anat. 



j See Burnett, loc. cit, p. 353; Zencker, tiber 

 die Daphuoidae in Muller's Archiv., 1851, p. 112; 

 and Leydig, tiber Artemia salina und Branchipus 

 stagnafis, in Zeitsch. fur Wissen. ZooL, voL iii. 

 1851, p. 297. 



Suctoria, terricola, errantia, and tubicola. 



|| For a clear and comprehensive account of the 

 reproduction of the Annelida in general, and with 

 special reference to the genus Hermella, one of the 

 suctorial Annulata, the excellent memoir of Quatre- 

 fages, in the Annal. des Scien. Nat., 1848, vol. x. p. 

 153, in which, in addition to his own researches, 

 are duly recorded those of previous observers. 



\ Zeitsch. fur Wissen. Zool., vol. i. p. 123. 



which have not as yet been referred to any 

 general law. In the ova of these animals the 

 covering is double, consisting of a delicate in- 

 ternal vitelline membrane, and an external 

 envelope or chorion, to which a layer of dis- 

 tinct flattened and nucleated cells is adherent; 

 and within the vitelline membrane there is a 

 collection of nucleated cells which displace 

 and partially surround the usual finely granu- 

 lar or formative yolk-substance. Leuckart* 

 informs us that the same peculiarity exists in 

 Pontobdella; but the nature and destination 

 of this inner cellular part of the ovum does 

 not appear as yet to be understood in either 

 of the animals mentioned. 



In the Piscicola, Leydig observed the ovum, 

 while within the ovarian cavity, to be com- 

 pletely surrounded for a time and enclosed by 

 a consistent mass or covering of spermato- 

 zoa; and it has been observed that in this 

 animal the germinal vesicle has not in general 

 disappeared till some time after the ovum has 

 thus encountered and been enveloped by the 

 mass of spermatic substance. 



In the Lumbricus, Meissner-f- has made the 

 novel and interesting observation, that pre- 

 vious to the encounter of the spermatozoa 

 with the ovum, the latter loses the vitelline 

 membrane which before covered it, and that 

 the spermatozoa then penetrate, in great 

 numbers, the whole surface of the exposed 



yolk. 



Fig. 82*. 



Ova of tfie Lumbricus during fecundation. (From 



The figures represent three views of the ova of 

 Lumbricus agricola, a. b. on their flat sides, c. seen 

 edgeways. Over the surface spermatozoa are seen 

 penetrating the vitelline substance, giving to it on 

 a large scale the appearance of a ciliated surface. 

 The ovum which has now reached the receptaculum 

 seminis is without vitelline membrane, the yolk 

 being thus directly exposed to the action of the 

 spermatic masses ; but the vitelline membrane ex- 

 isted at an earlier period and disappeared by solu- 

 tion in the course of the descent of the ovum. 



The development of the ova in Hermella 

 has been minutely described by Quatrefages ; 

 and this may be taken as an example of the 

 general nature of this process among the 



* Article Zeugung, p. 809. 

 t On the penetration of spermatozoa, &c., in 

 Zeitsch. fur Wissen. Zool. voL vi. 

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