OVUM. 



[ 670 ] 



OVUM, 



becomes covered with cilia, and undergoes 

 a regular motion of rotation; and certain 

 movements and changes in form of the 

 yolk-substance, which forms Amoeba-like 

 processes, have been noticed. In the un- 

 impregnated ovum, decay and decomposi- 

 tion subsequently take place. 



The essential part of the process of im- 

 pregnation consists in the penetration of 

 the yolk by the spermatozoa, and their 

 subsequent solution in it. This takes place 

 either through the micropyles or the radiate 

 canals, or directly into the naked yolk. 



In the impregnated ovum, the germinal 

 vesicle soon disappears, the chorion becomes 

 thinner, the ovum grows, and the yolk be- 

 gins to undergo segmentation ; but just 

 before this process commences, one or two 

 globules separate from the substance of the 

 yolk, being apparently pressed out of it, 

 and occupy the interspace between the yolk 

 and the chorion; these globules subse- 

 quently dissolve in the liquid. 



In the process of segmentation, at first a 

 notch or slight indentation appears on some 

 part of the surface of the yolk; this be- 

 comes deeper and deeper, so as to encircle 

 the yolk with an annular depression. Soon 

 after the commencement of this, a clear 

 spot appears in the centre of each circum- 

 scribed portion of the yolk. The depression 

 becoming deeper, the yolk is divided into 

 two distinct portions. The process is con- 

 tinued in the case of each of these in 

 exactly the same manner, and in that of 

 the segments arising from their subdivision 

 also, each simultaneously acquiring a clear 

 spot, until the yolk appears entirely com- 

 posed of innumerable small bodies having 

 the appearance of nucleated cells. Finally 

 these become very minute, and the yolk 

 acquires much the appearance it had before 

 impregnation. Cells then form in the yolk, 

 as in an ordinary blastema, from without 

 inwards, and from the spot originally occu- 

 pied by the germinal vesicle as a centre ; 

 and from these the tissues of the embryo 

 are formed. 



According to this description, the seg- 

 mentation is not a process of cell-division 

 or endogenous cell-formation, and the nu- 

 clear spots would correspond to portions of 

 the yolk substance from which the granules 

 and globules of sarcode were absent. Com- 

 pare p. 140. 



In unimpregnated ova, segmentation takes 

 place to a certain extent, but irregularly and 

 incompletely. 



In the impregnated ova of some animals, 

 as in certain of the Batrachia, most fishes 

 and Cephalopods, the segmentation is only 

 partial, a portion of the yolk remaining as 

 at first. 



In some of the Mammalia, the zona pel- 

 lucida is traversed by very fine radiating 

 lines (canals), which are best seen in ova 

 immersed in water. 



In the lower Vertebrate animals, the ova 

 are often covered by new layers, secreted 

 by the ovaries, as in the Batrachia (frog, 

 &c.), where a thick gelatinous coat is pre- 

 sent. In the osseous Fishes, the vitelline 

 membrane is frequently elegantly sculp- 

 tured, and finely and closely punctate from 

 the existence of minute canals traversing 

 its substance. A second coat is also pre- 

 sent, and sometimes a third or albuminous 

 layer. In many of the Cyprinoidea, this 

 layer is represented by small radiate cylin- 

 ders. In several Fishes, as is so general 

 amongst the Invertebrata, especially Insects 

 (Eoos), the vitelHne membrane or chorion 

 exhibits a facetted or sculptured appearance, 

 derived from the impression of the epithe- 

 lium lining the ovarian passages. 



In addition to the fine canals traversing 

 the membranes of the ovum, one or more 

 large canals or apertures are frequently met 

 with resembling the micropyles of vegetable 

 ovules, and receiving the same names; these 

 are most distinct in the ova of fishes and 

 insects. 



The study of ova and their changes is 

 very difficult. The most favourable objects 

 for the purpose exist perhaps in those of the 

 aquatic Mollusca ; the ova of insects, as the 

 large species of Musca, of species of Pule.r, 

 &c. are also easily accessible. Some im- 

 portant results have been obtained with the 

 ova of the frog (frog's spawn). 



BIBL. Kolliker, Mik. An. ii. ; Thomson, 

 Cycl. An. art. Ovum ; Newport, Phil. Tr. 

 1861 and 1853; Siebold, VergLAn.; Meiss- 

 ner, Sieb. $ Kottik. Zeitsch. vii. 208, 272 ; 

 Leuckart, Mull. Archiv, 1855 ; Claparede, 

 Bibl. Univ. Gentve, 1855; Ann. N. H. 

 1856, xvii. ; BischoiF, Sieb. $ Koll. Zeit. vi. 

 377; Radlkofer, Befruchtungs. 1857; Bene- 

 den & Bessels, M. M. J. 1869, 41 ; Kupffer, 

 M. Mic. Jn. 1869, 47 ; Kowalevski, Mem. 

 St. Petersb. xvi. 1871 ; Klein, M. M. J. vii. 

 1872, 193 ; Beale, Tr. Mic. Soc. 1867; How 

 Sfc. 307; Beneden, FCEuf, 1870; Dallinger 

 & Drysdale, M. M. Jn. xviii. 86 ; Schafer, 

 P)-. Roy. Soc. no. 168 ; Kolliker, Entwickel; 

 Balfour, Comp. Embryol. j M. Baker, Phys. 



