OVUM. 



[125] 



the proglottis ; and finally they are dis- 

 charged from this, usually after the separation 

 of the joint from the main tapeworm, by the 

 irregular rupture of the outer wall, or by a 

 genital aperture. Here, then, we have another 

 instance of the combi nation of the several com- 

 ponent elements of the ovum together with 

 the sperm, previous to the enclosure of the 

 whole by a membrane so as to give the form 

 of a complete ovum. 



The ova of most of the Cestoidea, as in the 

 common tapeworms, are of proportionally 

 small size. The external envelope is firm, 

 thick, and nearly homogeneous ; sometimes, 

 however, presenting a slight appearance of 

 fine radiated strias passing through it, which 

 recalls the structure of the thick membrane 

 of the Fish's ovum. The vitelline substance 

 is very finely granular, or almost clear ; the 

 germinal vesicle is perceived with difficulty, 

 but is of large size.* In some Cestoids the 

 external envelope is of a brown colour, as in 

 the Trematoda, and in others presents pecu- 

 liar forms and prolongations from its surface. 

 A delicate vitelline membrane is described 

 within the outer covering by some authors.f 



The segmentation of the yolk appears to be 

 complete ; but this process has been observed 

 only in a few instances. 



Of the ova of the Cystic Entozoa nothing 

 need here be said, seeing that it has already 

 been shown that the several genera of this 

 order, viz., Cysticercus, Ccenurus, and Echi- 

 nococcus, are only larral and aberrant forms of 

 the Cestoid worms, and being immature 

 animals, never produce ova, excepting through 

 their more advanced stage of cestoid develop- 

 ment. 



Echinodermata. The different orders and 

 families of this class are all of distinct sex, so 

 far as is yet known, with the single exception 

 of one of the Holothurida, viz., Synapta(S. 

 Duvernaea), described by Quatrefages^l as 

 presenting a combination of the testicles and 

 ovaries in one organ, resembling in some 

 measure that which exists in the Gasteropo- 

 dous Mollusca. 



In the females of Echinus, Asterias, and 

 Holothuria, the ova have been studied with 

 care by different observers. In all of them 

 the ova present, when mature, more or less of 

 a deep yellow, orange, or red colour, which 

 belongs to the yolk-substance. This sub- 

 stance is finely granular, and is enclosed, at 

 least in some, as Echinus, by a delicate 

 vitelline membrane ; but in others, as Holo- 

 thuria, there is a considerable deposit of an 

 albuminous layer of a peculiar structure, 

 which, from its adhering closely to the vitel- 



* See Kolliker in Mullet's Archiv. for 1843, 

 p. 92 ; Tafl. vii., fig. 44. 



f Details as to the structure of these ova will be 

 found in the work of Von Siebold in Burdach's 

 Physiologie, vol. ii. ; in Dujardin's Hist. Nat. des 

 Helminthes, see pi. ix. and xii. ; in Blanchard's 

 memoirs in the Annal. des Scien. Nat. for 1848, 

 p. 321 ; in Van Beneden's work ; and in Kuchen- 

 meister's more recent Handbuch der Parasiteu des 

 Menschen, &c., Leipzig, 1855. 



J Annal. des Scien. Nat., 1842, xvii. 



line membrane, obscures the latter envelope, 

 and thus has made its existence doubtful to 

 some observers. This albuminous deposit 

 also exists in Echinus, but is in that animal 

 distinguishable from the vitelline membrane.* 



The colour and opacity of the yolk-sub- 

 stance in the mature state of the ovum 

 usually prevent our perceiving the germinal 

 vesicle ; but in the earlier stages of formation, 

 when the ovum is of lighter colour or even 

 quite clear and transparent, a germinal vesicle 

 with a single distinct macula is easily per- 

 ceived. This vesicle has disappeared in the 

 ova which are deposited. The segmentation 

 of the yolk is complete in the Echinodermata : 

 the process has been fully traced by Sars in 

 Asterias f, and by various observers in some 

 other genera. 



It was in the ovum of Holothuria tubulosa 

 that Professor Johannes Muller first made the 

 novel and interesting discovery of an aperture 

 leading through the thick external membrane 

 towards the yolk ; an observation which has 

 been confirmed by various other physiologists J, 

 and has been productive of important con- 

 sequences in its extension to a number of 

 other animals in which such an aperture was 

 not previously suspected to exist. MUller 

 brought this observation before the Berlin 

 Academy, and it was noticed in the printed 

 report of the proceedings in 1851. A more 

 detailed account of his observations on this 

 subject is given by Muller in his Archiv. for 

 1854 (p. 60.). The very thick covering of 

 the ovum of Holothuria presents an appear- 

 ance of radiated lines running through it per- 

 pendicularly to the surface, which resembles in 

 some degree the marking in the membrane 

 of the Fish's ovum, but is not so distinct, and 

 does not appear, as in it, to be produced by 

 visible tubes or pores passing through the 

 membrane. The canal of the micropyle pierces 

 the whole thickness of the radiated membrane ; 

 but Muller conceived that it did not perforate 

 the delicate vitelline membrane placed on its 

 inner surface. Leydig, however, and Leuck- 

 art are of opinion that the canal passes com- 

 pletely into the interior of all the egg-coverings, 

 and reaches the surface of the yolk, so that it 

 may convey the spermatozoa to that body. 

 The entrance of the spermatozoa has not, 

 however, as yet been actually observed. 



According to Leydig, the thick membrane 

 may consist of several layers united together, 

 such as, internally the vitelline membrane, the 

 thick albuminous part in the middle, and ex- 

 ternally the nucleated layer formed by the 

 remains of the ovarian capsule. Leuckart and 

 Leydig have also pointed out the fact that the 

 formation of the canal of the micropyle in the 

 egg of Holothuria proceeds from or is con- 

 nected with the original attached and pedicu- 



* Derbes, in Annal. des Scien. Nat. 1847, 3 e Se'r. 

 vol. viii., p. 80, and Leydig in Muller's Archiv. for 

 1854, p. 312. 



f Wiegmann's Archiv. 1844, and Annal. des 

 Scien. Nat, 3 e se'r., vol. ii. p. 190. 



J Leuckart in Bischoff's Wiederlegung, &c., 1854, 

 and Leydig, loc. cit. 



