518 THE HUMAN BODY 



them enters the ovum, when it grows and forms the male pronu- 

 cleus (mn, Fig. 156). This travels towards the nucleus of the ma- 

 tured ovum or female pronucleus, fn, and in each pronucleus a 

 karyoplastic filament forms and breaks up into a set of V's; in the 

 pronuclei represented in Fig. 156 this has not yet taken place, the 

 karyoplasm being still arranged in a reticulum. The tail of the 

 spermatozoon (which represents, it will be remembered, the pro- 

 toplasm of a male cell) disappears; whether it is cast off when the 

 head enters the vitellus or mingles with the protoplasm of the latter 

 is not known. As the pronuclei approach one another two attrac- 

 tion particles, p, p, appear in the protoplasm of the ovum ; around 



these the granules of the vitellus show 

 a radial arrangement and a nuclear 

 spindle (p. 21) unites them. The 

 spindle lies with its long axis at right 

 angles to a line joining the pronuclei. 

 The latter next completely fuse 

 across the middle of the spindle and 

 form a new single nucleus. Fertili- 

 zation is then complete, and the ovum 

 capable of dividing or segmenting 

 (Fig. 11) to form the cells which 

 FIG 156. An ovum shortly be- by multiplication and differentiation 



fore the fusion of the pronucleu * 



a, zona peiiucida; 6, polar giobu- build up the embryo. The zona pel- 



les; fn, female pronucleus: mn. -, . -, , , , . ,, 



male pronucleus; pp, attraction lucida takes no part in the segmen- 

 tation and is gradually absorbed. 



which have not taken part m fer- Impregnation. The fertilized ovum 



tilization. 



continues its descent to the uterine 



cavity, but, instead of lying dormant like the unfertilized, seg- 

 ments (p. 30), and forms a morula. This, entering the womb, 

 becomes embedded in the soft, vascular mucous membrane from 

 which it imbibes nourishment, and which, instead of being cast off 

 in subsequent menstrual discharges, is retained and grows during 

 the whole of pregnancy, having important duties to discharge in 

 connection with the nutrition of the embryo. 



Sexual congress is most apt to be followed by pregnancy if it 

 occur immediately after a menstrual period; at those times a ripe 

 ovum is usually in the Fallopian tube, near the upper end of which 

 it is probably fertilized in the majority of cases. There is some 



