660 



THE HUMAN BODY. 



before reaching the Fallopian tube and fails to enter it, giving 

 rise to an extra-uterine pregnancy. 



The actual process of the fertilization of the ovum has only 

 been observed in the lower animals, but there is no doubt that 

 the phenomena are the same in all essentials in all cases. Some 

 of the spermatozoa penetrate the zona pellucida and the head 

 of one of them enters the ovum, when it grows and forms the 



male pronucleus (mn, Fig. 192). 

 This travels towards the nucleus 

 of the matured ovum or female 

 pronucleus, fn, and in each pro- 

 nucleus a karyoplastic filament 

 forms and breaks up into a set 

 of V's; in the pronuclei repre- 

 sented in Fig. 192 this has not 

 yet taken place, the karyoplasm 

 being still arranged in a retic- 

 ulum. The tail of the sperma- 

 tozoon (which represents, it will 



i^ rmn<miTwiW1 fliA rkvnfnrlocTYi 

 * 3a ? T 



of a male cell) disappears; 

 whether it is cast off when the 

 head enters the vitellus or min- 

 gles with the protoplasm of the 

 latter is not known. As the pronuclei approach one another 

 two attraction 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. 19) 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 by multiplication 

 and differentiation build up the embryo. The zona pellucida 

 takes no part in the segmentation and is gradually absorbed. 



The Signification of the Polar Globules. The union of 

 the male and female pronuclei is the essential fact in fertiliza- 

 tion and the material basis of all the phenomena of heredity; 

 therefore everything pertaining to it is of very great interest. 

 There is reason to believe that each half of the nucleus of the 

 fertilized egg contains karyoplasm from both pronuclei, and 

 that in all subsequent cell-divisions each new cell gets nuclear 



FIG. 192. An ovum shortly before 

 the fusion of the pronuclei. a, zona 

 pellucida; 6, polar globules; fn, female 

 pronucleus; nin, male pronucleus; pp, 

 attraction bodies, with the nuclear 

 spindle lyin between them; s, sper- 

 matozoa which have not taken part in 

 fertilization. 



