OVULITES. 



[ 509 ] 



OVUM. 



BiBL. Brown, Ajyp. to King's Voyage, 

 1820; Linn. Tr. 1833; Mirbel and Sparh, 

 Ann. Sc. Nat. 2 s6r. xx. 257 ; Mohl, Bof. 

 Zeit. 1847, 1855; Miiller, ibid. 1847; 

 Sclileiden, Nova Acta, xix. 29; Ormuh. 

 Bot. ; Hofmcister, Entsteh. d. Emhnjo, 

 1849; Unters. hoher. Crypt. 1851; Ahh. 

 K. siichs. Ges. vi. ; Planstein, Flora, 1857 ; 

 Tulasue, Aim. Sc. Nat. 3. xii. 21, 4. iv. 65 ; 

 Schaclit, Beitr. z. An. u. Phys. 1854 ; 

 Jahrh. loiss. Bot. i. 1857 ; Bot. Zeit. 1858 ; 

 Criiger, Bot. Zeit. 1851, 1856; Eadlkofer, 

 Entsteh. des Embryo, 1856; Henfi-ey, Linn. 

 Tr. xxi. 7, xxii. 69 ; Tr. Brit. Asxoc. 1856 ; 

 Bentley, Man. Bof.; Henfrey-Masters, i?oi^. ; 

 Cbalin, Ann. Sc. N. xix. 1874, 1 ; Sachs, Bot. 

 558; Stvasshm-gev, Zellcnbiidung, 1880. 



OYULI'TES, Lamarck {Oveolites) .—A 

 large elegant one-celled ForaLuiuifer, re- 

 garded by some as a calcareous Alga ; either 

 ovoid, sausage-shaped, or like a drumstick ; 

 shell porous, with large terminal apertures ; 

 length 1-25" and more. 



Fossil ; abimdant in the Eocene of Grig- 

 non, Hauteville, &c., France; rare in the 

 Miocene of San Domingo. 



BiBL. Parker & Jones, Ann. N. H. 3. v. 

 292, and 1877, xx. 77. 



OVUM OF Animals. — Several points in 

 regard to the structure of the ovum, and 

 the nature of the changes which it imder- 

 goes at different periods of its development, 

 are in doubt and obscurit3^ 



The first perceptible trace of the ovum 

 existing within the ovary is formed by a 

 very minute gi-anule or globule, not sur- 

 roimded by a cell-wall. This gradually 

 enlarges; and when it has attained a certain 

 size, being still very minute, a smaller sphe- 

 rical globule forms in its interior. The 

 minute internal globule is th.Q germinal spot; 

 and the external globule is the germinal 

 vesicle. It appears, however, that in some 

 cases the germinal spot is formed fu-st, and 

 the germinal vesicle subsequently. When 

 these have still further grown, a cell-waU 

 separated by a slight interspace forms 

 around the germinal vesicle ; and this inter- 

 space contains a transparent liquid. Minute 

 granules then arise in the liquid, which 

 becomes inspissated ; and subsequently a 

 number of globules of sarcode — yoJk- 

 glohides — become perceptible in it ; this 

 mass forms the yolk ; and the surrounding 

 membrane is the vitelline membrane. As 

 the ovum attains further development, al- 

 buminous layers are deposited upon and 

 fused with the vitelline membrane, fonning 



the zona pellucida or chorion (fig. 562 a), 

 Fig. 552. 



Human ovum from a Graafian vesicle of moderate 

 size, a, zona pellucida; ft, vitelline membrane and 

 outer boundary of the yolk ; c, germinal vesicle with 

 the germinal spot. 



Magnified 250 diameters. 



which appears as a white ring. The yolk- 

 globules are sometimes transparent, or 

 slightly granular; at others they contain 

 one or several vacuoles, and are frequently 

 aggregated into little groups. The yolk, 

 as it approaches maturity, frequently be- 

 comes coloured. It is usually whitish or 

 pale yellow in the Mammalia, Reptiles, and 

 Fishes, bright yellow or reddish in many 

 Birds, and often green, blue, violet, or red 

 in the Invertebrata. In the yolk of the 

 ova of reptiles and fishes, crystalline plates 

 are met with, consisting of an albuminous 

 substance, allied to Htematoidine. 



Viewing the ovum as a simple cell, the 

 germinal spot represents the nucleolus, the 

 germinal vesicle the nucleus, the vitelline 

 membrane or zona pellucida the ceU-wall, 

 and the yolk the cell-contents. 



Some authors consider that the vitelline 

 membrane is formed after the chorion. 



The ovum of man and the mammalia 

 differs from that of the lower animals in its 

 remarkably small size, which depends upon 

 the extremely small quantity of yolk enter- 

 ing into its composition. The mature ovum 

 of man and mammalia averages about 1-200 

 to 1-150" in diameter, being rarely 1-100". 

 Another peculiarity consists in their ova, 

 instead of being in immediate contact by 

 means of their chorion or outer envelope 

 with the stroma of the ovary, or loose 

 within the cavity of the latter, as in other 

 animals, being enclosed in distinct larger 

 cells — the Graafian vesicles. 



On the escape of the ovum from the ovary, 

 the phenomena whieh ensue vary according 

 to whether the ovum has been impregnated 

 or not. In both cases the germinal vesicle 

 and spot disappear ; an interspace, filled 

 with albuminous liquid, occurs between the 

 yolk and the zona pellucida ; the ovum 



