CHAPTER III. 



THE HUMAN EMBRYO. 



Our knowledge of the early stages of human development is very imperfect. 

 Upon the fertilization and segmentation of the ovum in man there are no obser- 

 vations whatever at present. It is not even known exactly how long the ovum 

 requires for its passage through the Fallopian tubes. The earliest stage of which 

 we have any comparatively adequate account is that represented by the ovum 

 described by H. Peters in 1899. A number of human embryos in various early 

 stages after the formation of the medullary canal and up to the stage with four 

 aortic arches have now been reported and studied ; some few of them thoroughly 

 and carefully. 



Calculation of the Age of the Human Embryo. 



The age of the embryo must be reckoned from the date of the fertilization of 

 the ovum, which presumably occurs in man in the upper third of the Fallopian 

 tube. It may be that ova become fertilized at various epochs, but fail to con- 

 tinue their development except when the fertilization occurs at the beginning of 

 a menstrual period. Ovulation occurs at all periods, but most frequently about 

 the time of menstruation, which is the expression of structural changes in the 

 uterus which enable the ovum to implant itself in the uterine wall. Hence only 

 when fertilization coincides with the beginning of menstruation can conception 

 follow with the result that the menstrual flow is stopped. Accordingly, the age 

 of the embryo is usuallv to be reckoned from the date of the beginning of the 

 first menstrual period which has lapsed. 



Experience, however, shows that sometimes conception occurs without 

 stopping the menstrual change at the time, but eliminating only the subsequent 

 periods, and in such cases the age must be estimated from the beginning of the 

 last menstruation. In the two cases the age of the embryo would differ by a 

 month (twenty-eight days), and this difference is so great that it obviates errors 

 of estimate. 



Up to the end of the ninth week the form and size of the embryo exhibit a 



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