[126] 



OVUM. 



lated condition of the ovum in the ovary, that 

 it is in fact the remains of the divided pedicle 

 after the ovum is separated from the place of 

 its original formation. 



serve 



Ovum and Micropyle in Holoihuria tubulosa. 

 (From Ley dig.} 



a, b. A small portion of the ovary from the 

 inner surface, containing ova in various earlier 

 stages of their development ; three of them project 

 from the inner surface, of which a is the most de- 

 veloped. In this one the pediculated attachment 

 and enclosure of the ovum by the nucleated ovarian 

 membrane is seen, the yolk granules and the ger- 

 minal vesicle with its macula. 



c. A more advanced ovum now separated from 

 the ovary. Externally the nucleated remains of 

 the ovicapsule are represented ; inside this the 

 thick albuminous layer marked with radiated lines, 

 and lined closely by the vitelline membrane ; both 

 these, as well as the ovicapsule, being perforated by 

 the micropyle formed at the place where the pedicle 

 formerly existed. 



The micropyle "aperture has also been ob- 

 irved in other Echinodermata, viz. by J. 

 Mliller in Ophiothrix fragilis, in which he 

 states its diameter to be yaW* anc * by n * s 

 son Max Miiller in Sternaspis thalassemoides.* 

 This aperture has not yet been observed in 

 the ovum of Echinus. In the fecundated 

 ova of this genus, however, Derbes observed 

 spermatozoa to have passed through the thick 

 external albuminous covering, but not within 

 the more delicate vitelline membrane ; but in 

 this animal the external covering is more like 

 a layer of soft albumen than a dense mem- 

 brane as in Holothuria. 



The ova of Echinodermata take their origin, 

 like those of other animals, by the formation 

 of the germinal vesicles. These have been 



* The micropyle was represented in the ovum of 

 Holothuria tubulosa by R. Wagner in his Icones 

 Zootomicae, tab. xxxii., fig. 12., before its nature 

 was known. The first discovery of a micropyle in 

 the animal ovum is therefore due to J. Miiller. The 

 next observations of a similar nature are those of 

 Leuckart and Keber. 



observed by Leuckart in the Holothuria tu- 

 bulosa, beginning to be formed in the ovarian 

 substance, which they cause to bulge or pro- 

 ject when they enlarge, so as to hang into the 

 ovarian cavity. The yolk-granules then come 

 to be deposited round the vesicles, rendering the 

 ova opaque, but during all this time the ovum 

 is attached and enveloped by the original 

 capsule derived from the ovary ; the albu- 

 minous layer is then deposited, and the ovum 

 being detached, the micropyle remains, as al- 

 ready stated, as the perforation in the pedicle 

 of attachment.* 



Polypma. Although the greater number 

 of the Polypi are commonly multiplied by a 

 process of gemmation, as has already been 

 stated in a former part of this article, yet they 

 are all capable of attaining sexual complete- 

 ness, and are also reproduced by means of 

 fecundated ova. From the varieties, however, 

 presented by the form both of the gemmules 

 and true ova in different genera of Polypes, 

 considerable difficulty has been experienced 

 in determining the exact circumstances in 

 which the ova are produced, and the distinc- 

 tion between the germs from which true ova 

 and those from which gemmae are formed. 

 This is more especially the case among the 

 ciliobrachiate Polypes or Bryozoa, which in 

 their general organisation approach very nearly 

 the tunicate Mollusca, but which in their mode 

 of reproduction resemble closely some of the 

 Polypes. 



The ova of the common Hydca, already re- 

 ferred to in a previous part of this article, 

 present the character common to the class, of 

 being enveloped by a firm covering or shell 

 membrane, which seems to be formed from 

 modified cells, and which is sometimes beset 

 with rough processes or projecting bristles or 

 barbed spines somewhat like those of the 

 Bryozoa. 



In the Tubularidae and Sertularidae the 

 ova are formed in ovigerous capsules, which 

 may be regarded as modified individuals or 

 polype-heads of the compound animal formed 

 by gemmation. In some instances these are 

 detached from the parent stem, as in Tubu- 

 laria indivisaf ; in other genera they remain 

 attached, and their ova, or the ciliated em- 

 bryos developed from them, are discharged 

 from the cavities in which they are formed J ; 

 but as the phenomena of the production of 

 these ova have been fully described by Pro- 

 fessor Rymer Jones in the article POLYPIFER A, 

 it is unnecessary to enter into farther details 

 with regard to the process in this place. 



* In addition to the memoirs previously quoted, 

 descriptions of the ova of Echinodermata will be 

 found in the following : viz., those of Comatula by 

 J. Muller, in Mem. of the Berlin Academy for 

 1841 ; of Asteracanthion, in Wagner's Prodromus, 

 and in the 5th Part of Carus and Otto's Tabulae 

 Anat. Compar. ; those of Echinus by Derbes, loc. cit. ; 

 and by Krohn in Beitr. zur Entwick. der Seeigel- 

 larven, Heidelberg, 1849, &c. 



f Sir John Dalzell, Remarkable Animals of 

 Scotland, &c. 



J Dumortier and Van Beneden's Researches, in 

 Mem. of the Acad. of Belgium, 1842, torn. xvi. 



