AGE OF HUMAN EMBRYOS 401 



To my great surprise and pleasure I found that these authors 

 had reached a conclusion similar to mine; namely, that the age 

 of young embryos is no longer to be computed according to the 

 convention of His. They not only give a detailed and excellent 

 account of their own specimen, but also reconsider all other cases 

 of young specimens in relation to their age, which have been 

 published by well-known writers. They assume that the copula- 

 tion age is probably very nearly the true age of embryos, and 

 that henceforth we will have to consider the question from this 

 standpoint. 



According to Bryce and Teacher, it is now generally admitted 

 that the menstrual cycle in man and monkeys is homologous 

 with the oestrus cycle of the lower mammals. The oestrus cycle 

 is divided by Heape into pro-oestrum, oestrus and dioestrum, 

 and this division has been confirmed for many mammals by 

 his own researches and those of F. H. A. Marshall. During pro- 

 oestrum the generative organs of the female show signs of special 

 activity, such as swelling of the vulva, coloration or flushing of 

 the surroundings, and a discharge of blood or mucus from the 

 vagina. This is immediately followed by the 'oestrus,' or 'period 

 of desire,' during which alone the female is capable of impregna- 

 tion and will receive the male. If pregnancy does not occur, 

 oestrus, after a brief space in which desire subsides (metoestrum), 

 is succeeded by a period of quiescence or dioestrum, which lasts 

 till pro-oestrum again sets in. In polyoestrous mammals several 

 cycles of this kind may follow one another. Menstruation in 

 the human female is homologous with pro-oestrum, as first 

 pointed out by Heape. Though there is no fixed 'period of 

 desire' there is an indication that a vestige of this persists, in the 

 fact that a phase of more pronounced oestrus commonly succeeds 

 the cessation of menstruation. This view is confirmed by our 

 records, for we frequently hear from a patient that a fruitful 

 copulation occurred shortly after the menstrual period; and it 

 may be that this opinion records also the rupture of the Graafian 



