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 tube. The earliest stages of which 

 we have comparatively adequate accounts are those represented by Peters's ovum 

 (1899) and Herzog's (1909). 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 continue 

 their development except when the fertilization occurs at the beginning of a men- 

 strual 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 usually 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 

 correlated development, so that generally an embryo at a given stage of develop- 

 ment in form will agree with its fellows in size; but to this rule there are not in- 

 frequently exceptions, and sometimes an embryo is found much larger than others 



118 



