COMPOSITION OF THE HUMAN OVUM -i^jj 



and altered ovum is in a condition to assume the cleavage necessary to the formation of the blastodermic membrane, 

 which in the higher vertebrates is the starting-point for the embryo and foetus. The changes which supervene during 

 the fertilising process bear a certain analogy to those occurring in the foetus itself (Plate xcii., Fig. 1, E, F, G, H, I, 

 page 396), so that the scheme of reproduction is, from the first, involved and complex, and the outcome of pre- 

 arrangement and design. 



While the matrices of the two pronuclei appear to blend, their chromatin filaments maintain their identity. 



The chromatin filaments form two sets of looped-v-shaped fibres of two each, and as these ultimately divide 

 longitudinally they make two sets of four each. At one stage they flatten and arrange themselves transversely 

 across the ovum, and so participate in, if they do not actually cause, the division of the ovum into two. They 

 subsequently open out transversely, and then at right angles, and in conjunction with radiating, concentric, ovoid, 

 and circular arrangements in the attraction-spheres complete the division of the ovum. 



The several points discussed in connection with the maturation, discharge, and fertiUsation of the ovum are 

 illustrated at Plate xc, Plate xci., and Plate xcii.. Fig. 1, pp. 391, 393, and 396. 



§ 72. Parts Entering into the Composition of the Human Ovum. 



The human ovum in its entirety consists of the following parts : (1) globular zona pelluoida, otherwise called 

 zona radiata ; (2) globular vitelline membrane ; (3) globular vitellus or yolk, consisting of protoplasm and nutritive 

 material ; (4) globular germinal vesicle containing globular germinal spot. 



The same parts are encountered in the rabbit. 



The several parts of the ovum can only be made out by the aid of the microscope. In Plate Ixxxvi., page 383, 

 A, the ovum is seen under pressure and is more or less flattened. In B, C, the zona pellucida or radiata, 

 which has considerable resisting power, is purposely burst ; the protoplasm and nutritive material forming the 

 vitellus or yolk (6), and the germinal vesicle (c) having escaped. At D, the radiation in the zona pellucida (now 

 known as zona radiata) is indicated (a), as also the vitellus (6), germinal vesicle (c), and germinal spot {3,). The radia- 

 tion of the zona pellucida is beheved to contain pores, which permit the passage of noirrishment to the ovum and 

 provide entrances to the spermatozoa. The zona pellucida on leaving the Graafian foUicle is invested with a clear, 

 granular substance resembling the white of egg, which is supposed to contribute to the nourishment of the ovum, and 

 disappears when the ovum reaches the Fallopian tube. The protoplasm of the ovum on a casual examination is a 

 clear, apparently structureless, substance. On a closer examination, and when re-agents are employed, it is seen 

 to consist of a fine reticulum especially observable near the periphery of the ovum and around the germinal vesicle : 

 in other words, the protoplasm is differentiated. The other ingredients of the ovum (deutoplasm) contribute to 

 its nourishment. 



The germinal spot, which measures -^^ to yj„ of an inch in diameter, consists of a nuclear membrane 

 containing a clear material, with strands of karyoplasm enclosing one or more nucleoli. If only one nucleolus occurs 

 it is known as the germinative spot {macula germinativa) . A certain amount of differentiation is also traceable 

 in the germinal vesicle. The old idea, that protoplasm is identical and structureless, may now be regarded as 

 exploded. The amount of nutritive material suppUed to the ovum is less in mammals than in fishes, reptiles, and 

 birds, and Professor F. M. Balfour conveniently divided ova into those with (a) accumulation of nutritive or 

 yolk matter at one pole (telolecithal ova — fish, reptile, and bird among vertebrates) ; (b) those with the accumula- 

 tion of yolk in the middle ovum {centrolecithal ova — ^in arthropods) ; and (c) those with yolk scattered equally and 

 in small amount through the protoplasm of the ovum {alecithal — mammals, amphioxus, echinoderms). This 

 distribution of the nutritive material (proto- and deuto-plasm) determines to a large extent the nature of the 

 segmentation in the several ova. 



The development of the ovum after it leaves the ovary is at once intrinsic and extrinsic : intrinsic as regards 

 changes occurring in itself, and extrinsic as regards changes induced in it by the presence of the spermatozoon or 



male element. 



The intrinsic changes consist in the extrusion of certain constituents of the nucleus of the ovum known as the 

 " polar globules " : the extrinsic changes consisting in the intrusion into the nucleus of a spermatozoon or sperm- 

 cell to take the place of the extruded polar globules. The ovum or female element on its part makes room for the 

 spermatozoon or male element, and the latter takes full advantage of the arrangement. The male and female 

 elements come together, and either fuse and merge into each other's substance or, as B. v. Beneden thinks, they 

 maintain their identity, and mutually produce a double or compound cell, which is the starting-point of the new 

 being (Plate Ixxxvi., H to S inclusive, page 383). 



The fertilisation of the ovum, it will be seen, is brought about by a union of the most intimate kind ; the male 



3 B 



