IMPREGNATION 441 



undulations of this flagellum the sperm is forced ahead, the movements 

 being not too unlike those of a frog-tadpole when its tail is at its 

 longest. 



In the chapter on Protoplasm (see page 43) we suggested the various 

 forms of barotaxis, or reaction of certain organisms to pressure, and found 

 that rheotaxis, the reaction to a current of fluid, was a common cause of 

 movements in small animals. This rheotaxis apparently helps the sper- 

 matozoa to pass up through the Fallopian tubes to the ova or ovum. 

 As has been already said, the current of lymph in the tubes under the 

 influence of the cilia set always from the fimbrise to the uterus. If, then, 

 the sperms are negatively rheotactic (that is accustomed to oppose a 

 current rather than to go with it), we have a tentative explanation of how 

 they reach their goal many centimeters away from the uterine cervix, 

 where they are mechanically deposited in a preservative and stimulating 

 menstruum (Adolfi). Chemo taxis also may aid in this ascent, for in 

 some plants it is the chief means to fertilization. Hundreds at least 

 of the spermatozoa approach and surround the ovum, and the place of 

 meeting appears at present to be usually the upper part of the tube. 

 The time required for this passage is not known, but it is probably hours 

 at least and it may be a day or more. The sperms appear to travel in 

 their normal liquid about 150 mm. per hour, but we have no evidence 

 that in the actual and devious conditions of the vagina, cervix, uterine 

 body, and tubes they can make anywhere near this speed, especially 

 because of the strong contrary lymph-current in the 120 mm. or so of the 

 tube. As frequent ectopic gestations attest, the place of fertilization may 

 be on the surface of the ovary sometimes and probably in any other part 

 of the ovarian-uterine path. Inasmuch as the location of the ectopic 

 growth is determined by the arrest of the descending fertilized ovum 

 rather than by any condition of fertilization itself, the usual place of the 

 latter is very hard to learn either by this or other means. 



Oviposition is the passage of the fertilized ovum down the oviduct 

 (in woman the Fallopian tube) and its implantation in the uterine mucosa 

 ready to develop. In birds reptiles, etc., the egg when laid, compared 

 with the human egg, is independent, requiring only a proper environ- 

 ment. All placental animals, however, lay their eggs into the uterus, a 

 special organ of development and birth. 



Fertilization, as has been said, ordinarily occurs in the Fallopian tube 

 from a few hours after a fertile coitus up to perhaps even three and a 

 half weeks (Duhrssen). The average period after coitus that an ovum 

 is fertile is estimated by Issmer at sixteen days (McMurrich). It will 

 thus be seen that the time of conception (fertilization) is impossible 

 of exact calculation in the human female because so many of the facts 

 are unknown. One large group of cases indicated that after a single 

 coitus the average length of pregnancy was 275 days, and this implies 

 that in such, usually vigorous, cases fertilization is prompt after copula- 

 tion. Ordinary cases in which coitus may take place at all times in 

 relation to menstruation are much harder to calculate. The long life 



