156 TEXT-BOOK OF EMBRYOLOGY. 



2. The time the ovum leaves the ovary is determined by the menstrual 

 period, but the rupture of the (Graafian) follicle is not necessarily coincident 

 with the beginning of hemorrhage; it may occur two or three days before or it 

 may occur during hemorrhage. 



3. The egg is not capable of being fertilized at any point in its course from 

 the ovary to the uterus, but only in the upper part of the oviduct. 



4. The spermatozoa which have entered -the female sexual organs must 

 await the ovum in the upper part of the oviduct, and can retain their vitality 

 here for several days or possibly for several weeks; the time of cohabitation, 

 therefore, does not stand in direct relation to the age of the embryo. 



5. In the majority of cases the age of the embryo can be estimated from the 

 beginning of the first menstrual period which has lapsed. It is possible, how- 

 ever, for menstruation to occur after fertilization of the ovum. 



6. The age of the embryo can be expressed thus : age = X M, or age = 

 X M 28. X is the date of the abortion and M is the beginning of the last 

 menstrual period. The second formula is used where it is necessary to estimate 

 from the beginning of the first period which has lapsed. 



There is no doubt whatever that the age of the embryo must be dated from 

 the time of fertilization of the ovum; but owing to the fact that the time of 

 fertilization of the human ovum is not known, the exact age cannot be deter- 

 mined. Even when the date of coitus and the time of cessation of the menses 

 are known, the uncertainty regarding the time of ovulation and the time re- 

 quired by the spermatozoa to reach the upper end of the oviduct must be 

 taken into consideration. It is now generally conceded that ovulation and 

 menstruation are coincident in the majority of cases, but, on the other hand, 

 ovulation is known to occur sometimes independently of the menstrual periods 

 (see also p. 30). 



In addition to the uncertainty regarding the time when development 

 begins there is also an uncertainty as to the time when the embryo ceases to 

 develop. For in most cases the embryos are abortions and the death of the 

 embryo does not necessarily precede immediately its expulsion from the uterus. 



It is convenient, however, for practical purposes, to have some means of 

 approximating the age of an embryo. His' formulae serve to determine the age 

 within certain limits. It is obvious from these formulae that there is a possibility 

 of an error of twenty-eight days in the estimate. Yet in the earlier stages of 

 development (during the first three months) the error can be corrected after 

 examination of the embryo, since there is no difficulty in recognizing the differ- 

 ence, for example, between an embryo two weeks old and one six weeks old. 



LENGTH. Many German authors employ two different methods for 

 measuring embryos at different periods. One of these methods they use 

 in measuring embryos between 4 and 14 mm., when the body is much curved. 



