OVUM. 



[127] 



In Hydractinia rosea, Van Beneden ascer- 

 tained the existence of the germinal vesicle 

 and nucleus within the ova while still con- 

 tained in the capsule; and it appears that in 

 all true ova of the Hydrozoa the vitellus, 

 which consists of finely granular substance, 

 undergoes a complete segmentation in the 

 same manner as in other animals in which it 

 presents a similar structure. 



In the common fresh-water polype, in which 

 ovigerous capsules, or ova, and spermatic cap- 

 sules were found coexistent on the same in- 

 dividuals, I observed sometimes the spermatic 

 capsules brought into contact with the surface 

 of the ova by the bending round of the body of 

 the polype at the time when the spermatozoa 

 were being discharged. This took place pre- 

 vious to the formation of the firm external 

 covering ; but I could not determine whether 

 fecundation had thus taken place or whether 

 any spermatozoa had penetrated the ovum. 



In some of the Hydrozoa, as in the com- 

 mon green polype, the ova are single, while in 

 others as in Hydra fusca, figured by R. Wag- 

 ner*, there are several ova enclosed in the 

 same capsule. 



It is remarkable that, while in some Hy- 

 drozoa the ova are developed from animals 

 which retain the polype form in their com- 

 plete sexual condition, or from modified po- 

 lype heads, in others, as in Coryne, FritiU 

 laria and Campanularia dichotoma, it is only 

 from a medusoid progeny separated from the 

 polype stock that the true fecundated ova are 

 produced. 



In Anthozoa, the most of which, as Actinia, 

 Alcyonium, Veretillum, Gorgonia, and the 

 Corallines are hermaphrodite, the ova consist 

 of finely granular yolk, germinal vesicle and 

 macula, and undergo complete segmentation. 



The Bryozoa may be most appropriately 

 considered in this place, as they present con- 

 siderable analogy to the compound polypes 

 in the mode of their reproduction. They are 

 of separate sexes, and appear to be propagated 

 in three modes, viz. : 1st, by gemmation ; 

 2nd, by true fecundated ova ; and 3rdly, by 

 bodies which have long been regarded as ova, 

 but which according to Professor Allman's 

 recent researches may rather be considered as 

 peculiar encysted gemmnles, and may pro- 

 bably be analogous to the so-called winter 

 ova of Daphnia and Lacinularia to which 

 reference has previously been made. 



The development of the true ova of Pedi- 

 cellina observed by Van Beneden has been 

 already described.-f- In this instance the ova 

 are arranged in clusters surrounded by a 

 transparent capsule. In each ovum the finely 

 granular yolk undergoes a complete segmen- 

 tation. The germinal vesicle possesses a sin- 

 gle macula. 



According to Van Beneden and Dumor- 

 tierf, the ova of Alcyonella are developed in 

 ovarian sacs connected with the inner end of 



* Icones Zootomicse. 

 t See p. 23. andjfy. 19. of this article, 

 j Mem. sur les Polypes d'Eau douce. Acad. de 

 Belgique, 1842. 



the stomach. They are described as com- 

 mencing by the formation of germinal vesicles 

 with nuclei or maculae, and as having subse- 

 quently the granular yolk-substance deposited 

 round each vesicle ; and these authors de- 

 scribe the same ova as acquiring at a later 

 period the peculiar horny or cellular covering 

 which forms the two-valved shell membrane 

 long known as belonging to the winter ova 

 of this and several other genera of fresh- 

 water polypes. But with regard to the na- 

 ture of these bodies and the mode of their 

 formation some doubts may arise in conse- 

 quence of the researches of Professor Allman. 

 The bodies in question are at first nearly 

 spherical and of a light or milky colour ; they 

 become later of an oval form, and flattened or 

 discoid, and the cells of the shell -covering 

 are then developed, and acquire the deep 

 brown colour which very generally prevails 

 among these bodies when arrived at maturity, 

 and which makes it impossible to trace farther 

 the changes within the ovum. These cells 

 are developed to a greater extent round the 

 widest margin of the disc, so as to form there 

 a thick ring or border, which is afterwards 

 cleft in two when the valves of the shell open 

 to allow the escape of the embryo. 



The same authors have described the pro- 

 pagation of the Paludicella to take place in 

 summer by means of buds, and in winter by 



92*. 



Formation and Structure of the ova of Lophopus 

 Bakeri. (From Van Beneden.) 



These represent, according to Professor Allman, 

 not the true ova, but the Winter ova or " Stato- 

 blasts." a. The ovum previous to the deposit of the 

 cellular covering and marginal plate, b. This co- 

 vering now in progress of formation, c. and d. pro- 

 file and front view of the ovum, when completed, 

 showing the structure of the cellular border which 

 is afterwards cleft in two at the edge, when the era 

 biyo is about to escape. 



e. An ovum at an earlier stage showing the ovi- 

 capsule in part removed from one side of the ovum 

 and its cellular covering. 



