i6 



HUMAN ANATOMY. 



Fig. 12. 



Head 



Neck , 

 Connect- 

 ing piece 



TaiW 



general are found, including the all-important chromatin fibrils, nuclear matrix, and 

 nucleolus ; the latter, in the original terminology of the ovum, is designated as the 

 germinal spot, and measures about .005 millimetre in diameter. In addition to 

 these more easily distinguished components of the maternal cell, the centrosome 

 must be accepted as a constant constituent of the fully formed, but unmatured, 

 ovum, although its presence may escape detection. 



The Spermatozoon. — The male germ-cell, the spermatic filament, is produced 

 by the specialization of epithelial elements lining the seminiferous tubules within the 

 testicle. The human spermatozoon consists of three parts — the ovoid head, the 



cylindrical middle-piece, which includes the slightly-constricted 

 neck and the connecting -piece, and the attenuated and greatly 

 extended tail ; of these, the head and middle-piece are the most 

 important, since these parts contain respectively the chromatin and 

 the centrosome of the cells from which the spermatic filaments are 

 derived. The dentrosome is represented by two minute spherical 

 bodies, the neck-granules, which lie in the neck immediately 

 beneath the head, at the extremity of the axial fibre ; the latter 

 extends throughout the spermatozoon from the head to the termi- 

 nation of the tail, ending as an extremely attenuated thread, the 

 terminal filameyit. The tail corresponds to a flagellum and serves 

 the purposes of propulsion alone, taking no part in the important 

 changes produced in the ovum by the entrance of the male element. 

 Maturation of the Ovum. — Maturation, or ripening of 

 the ovum, is that process by which the female element is prepared 

 for the reception of the spermatozoon. It takes place, however, 

 entirely independently of the influence of the male or of the 

 probability of fertilization, every healthy ovum undergoing these 

 changes before it becomes sexually ripe. About the time that the 

 ovum is liberated from the ovary by the bursting of the Graafian 

 follicle, as the sac which encloses the &^^ within the ovarian 

 stroma is called, its nucleus engages in the complicated cycle 

 already described as mitotic division. The nucleus migrates to 

 the periphery of the ovum, loses its limiting membrane, and un- 

 dergoes division, one pole of the nuclear spindle being located 

 within the protrusion of protoplasm which has coincidently taken 

 place. With the division of the nuclear chromatin, the protruded 

 protoplasm becomes constricted and finally separated from the 

 ovum ; the minute isolated mass thus formed, containing one-half 

 of the maternal chromatin, is \^^ first polar body . Almost imme- 

 diately the mitotic cycle is repeated, and again results in the con- 

 striction and final separation of a minute cell, the second polar 

 body. These two isolated portions of the ovum remain visible for a long time as 

 small, deeply stained cells lying within the perivitelline space beneath the zona 

 pellucida. With each division of the egg-cell, one-half of the chromatin passes to 

 the polar body, the matured ovum consequently retaining but one-fourth of the original 

 chromatin. While the latter is thus diminished at each division, the masses of chro- 

 matin are reduced to one-half the normal quota of chromosomes, this reduction being 

 effected just before the first polar division. 



The chromatin remaining within the ovum after the repeated division becomes 

 collected within a new nucleus, which now takes a non-central position within the 

 ^<g^, and is henceforth known as the female pro?iucleus or egg-nucleus. After mat- 

 uration the ovum is prepared for union with the spermatozoon, although in many 

 cases the male sexual element has actually entered the ovum before the completion 

 of the maturation cycle : should, however, impregnation not occur, the ovum passes 

 along the oviduct into the uterus and is finally lost. The passage of the human 

 egg from the ovary to the uterus occupies, probably, about eight days, a period 

 corresponding closely to the length of time that the ovum retains its capability of 

 fertilization. 



The significance of the extrusion of the polar bodies — a process which occurs 



Terminal 

 filament 



1 



Diagram of human 

 spermatozoon; a, neck- 

 granules, representing 

 the centrosome; 6, axial 

 fibre, X i8oo. {Meves.') 



