CHAPTER IV 



PREGNANCY 



As already mentioned, the ovum is usually fertilised in the upper 

 or ovarian part of the Fallopian tube. The movements of the 

 cilia lining the lumen of the tube, assisted by rhythmical con- 

 tractions on the part of the muscular wall, propel the ovum along 



Fig. 34. — Segmentation of a nianimaUan ovum. 

 «, h, r, Earlier stages ; rf, c, morula stage ; />.//^., polar bodies ; z.p.^ zona 

 pellucida, or outer covering of the ovum. (After Allen Thomson, 

 from (^uaiii's Anatomf/.) 



the tube and into the uterine cavity. The complete passage of 

 the ovum through the tube is believed to take about seven days 

 in man, but in the lower mammals {e.g. the pig), in which the 

 question has been investigated, it almost invariably occupies four 

 days. During its passage the .ovum segments, that is to say, 

 it divides, first into two, and then into four, eight, sixteen, and 

 thirty-two cells, and so on until it forms a mulberry-shaped mass 



60 



