OVUM. 



[ 483 ] 



OVUM. 



needle), another slice is made so as to leave 

 a section preserving all the central part of 

 the ovule. This adheres either to the finger 

 or the razor, and a drop of water should be 

 placed on it to free it ; then it may be trans- 

 ferred to a slide with a very fine camel's- 

 hair pencil. Examined under a low power 

 (a half-inch), it will be probably found to 

 require further dissection, with exceedingly 

 fine needles, under a simple lens ; sometimes 

 mere pressure is of service. For the minute 

 details, the quarter and eighth object-glasses 

 will require to be applied. We have found 

 ovules which have been kept in spirit easier 

 to dissect ; when fresh, the cell-membranes 

 are excessively delicate. It need scarcely be 

 added, that ovules require to be examined 

 in all stages in order to understand their 

 developmental characters; and the student 

 must not be disheartened by the failure of a 

 large proportion of his sections to afford 

 satisfactory observations. 



BIBL. Works on Physiological Botany; 

 Amici, Ann. des Sc. nat. 3 ser. vii. p. 193; 

 Mohl, ibid. ix. p. 24; Miiller, ibid. p. 33 

 (abstracted from the Botanische Zeitung, 

 1847); Hofmeister, DieEnstehung des Em- 

 bryo, Leipsic,1849 (abstr. iuAnn. des Sc. nat. 

 3 ser. xi. 375), Vergleich. Untersuch. Leipsic, 

 1841, Flora, 1851. p. 450, 1855. p. 257; 

 Tulasne, Etudes, fyc., Ann. Nat. Hist. 2 ser. 

 i. p. 49, ix. p. 442, Ann, des Sc. nat. 3 ser. 

 xii. p. 21 ; Henfrey, Linnean Trans, xxi. p. 

 7; Schleiden, Enstek. d. Eichens, Nova 

 Acta, xix. p. 29, Grilndzuae, 3rd ed. Bd. ii. 

 p. 339 ; Schacht, Entwick. des Pflanzenemb. 

 Verh. Nederlandsch. K. Inst. 1850 (abstr. 

 Ann. des Sc. nat. 3 ser. xv. p. 80), Beitr. z. 

 Anat. u. Phys. Berlin, 1854, Flora, 1855. 

 p. 145; Criiger, Bot. Zeit. iv. p. 57 (1851); 

 Mohl, Botan. Zeit. xiii. p. 385 (1855). 



OVUM, OF ANIMALS. Several points 

 in regard to the structure of the ovum, and 

 the nature of the changes which it under- 

 goes at different periods of its development, 

 are in doubt and obscurity. 



The first perceptible trace of the ovum 

 existing within the ovary is formed by a 

 very minute granule or globule, not sur- 

 rounded by a cell- wall. This gradually en- 

 larges, and when it has attained a certain 

 size, being still very minute, a smaller sphe- 

 rical globule forms in its interior. The mi- 

 nute internal globule is the germinal spot, 

 and the external globule is the so-called ger- 

 minal vesicle. 



It appears, however, that in some cases 

 the germinal spot is formed first and the 



germinal vesicle subsequently. When these 

 have still further grown, a cell- wall separated 

 by a slight interspace forms around the ger- 

 minal vesicle, and this interspace contains a 

 transparent liquid. Minute granules then 

 arise in the liquid, which becomes inspissated, 

 and subsequently a number of globules of 

 sarcode become perceptible in it. This mass 

 forms the yolk. When this unimpregnated 

 ovum has attained considerable development, 

 it is found to consist of the following parts : 

 an outer structureless coat, sometimes of 

 considerable thickness, and then appearing 

 under the microscope as a white ring (fig. 

 55 7), the zona pellucida or chorion ; within 



Fig. 557. 



Human ovum from a Graafian vesicle of moderate size, 

 a, vitelline membrane or zona pellucida ; b, outer boun- 

 dary of the yolk and inner boundary of the zona ; c, ger- 

 minal vesicle with the germinal spot, 



Magnified 250 diameters. 



this the vitellus or yolk, consisting of a more 

 or less inspissated medium, and containing 

 granules and globules of sarcode, or con- 

 densed yolk-substance (b); the germinal ve- 

 sicle (c), and the germinal spot. The glo- 

 bules are sometimes transparent or slightly 

 granular, at others they contain one or seve- 

 ral vacuoles ; they are known as the yolk- 

 cells or globules, and are frequently aggre- 

 gated into little groups. The yolk, as it ap- 

 proaches maturity, frequently becomes co- 

 loured ; it is usually whitish or pale yellow 

 in the mammalia, reptiles, and fishes; bright 

 yellow or reddish in many birds ; and green, 

 blue, violet, or red in the invertebrata. 



According to the cell-theory, viewing the 

 ovum as a simple cell, the germinal spot re- 

 presents the nucleolus, the germinal vesicle 

 the nucleus, the zoiia pellucida the cell- wall, 

 and the yolk the cell-contents. 



At a subsequent period the yolk becomes 

 surrounded by a distinct, delicate, structure- 

 less cell-membrane, lying within the chorion, 

 and forming the vitelline membrane. 



The ovum of man and the mammalia dif- 

 fers from that of the lower animals in its re- 

 markably small size, which depends upon 

 the extremely small quantity of yolk entering 

 into its composition. The mature ovum of 

 2i2 



