598 THE HUMAN EMBRYO. 



The urino-genital canal shortens considerably in the female, 

 so as to bring the aperture of the urethra close to the surface. 



The above changes are usually completed in both sexes by 

 the end of the third month ; but they may be delayed until a 

 much later date. 



THE FCBTAL MEMBRANES AND THE PLACENTA. 



1. The amnion is the thin transparent membrane which 

 invests the embryo like a sac, covering its dorsal surface and 

 sides (cf. Pig. 197, an). 



The mode of formation of the amnion in the human embryo 

 has not yet been determined. In Reichert's ovum, estimated 

 to be twelve or thirteen days old, there was no trace of an 

 amnion present (Figs. 172 and 173); while in the embryos 

 E and SR (Figs. 176, 178, 179), which are believed to be of the 

 thirteenth day, the amnion is already fully formed. Figs. 186 

 to 188 show the mode in which the development of the amnion 

 is believed to occur, by growth backwards of a fold of the wall of 

 the blastodermic vesicle over the embryo ; but the figures are 

 purely hypothetical, and the intermediate stages which they 

 represent have not been seen. 



Of the two layers of which the amnion consists, the outer 

 one (cf. Fig. 188) is simply a part of the wall of the blastodermic 

 vesicle, and it is usual to limit the term amnion to the inner 

 layer, which more immediately invests the embryo. The space 

 between this inner layer, or amnion, and the embryo is spoken 

 of as the amnionic cavity, and is filled with fluid. 



The rate of growth of the amnion, as compared with that of 

 the embryo itself, varies considerably at different periods of 

 development. On its first formation, about the thirteenth day, 

 the amnion invests the embryo fairly closely (Fig. 179). Dur- 

 ing the third week the amnion grows rather more rapidly, so 

 that the space between it and the embryo enlarges somewhat 

 (Fig. 196). During the fourth week, as at the corresponding 

 stages in the rabbit or chick, the embryo grows considerably, 

 and at the end of the week the amnion invests it very closely. 



During the second month the amnion enlarges much more 

 rapidly, and the amnionic cavity becomes a space of consider- 

 able size, filled by the liquor amnii (Fig. 254). Owing to this 



