THE DEVELOPMENT OF THE EMBRYO 1135 



an animal in heat.^ Having established this fact, it may then be 

 proved that ovulation is not synchronous with menstruation. Thus, 

 it is well known that the Mosaic Law regards Jewesses unclean during 

 the menstrual period and for 7 days thereafter. In these women, 

 therefore, conception must take place after this period and before the 

 onset of the next menstrual flow. Moreover, Pinard^ has shown that 

 about three-fifths of the women who marry during the interim between 

 two menstrual periods and miss the subsequent flow, give birth to full- 

 term children 280 days after the beginning of the last menses. In 

 these cases, the duration of pregnancy is less than 9 calendar 

 months. Consequently, if ovulation takes place some time before 

 the onset of the menstrual flow, the latter must be in the nature of a 

 process of purification which prepares the endometrium for the suc- 

 ceeding ovulation. This cyclic regeneration, therefore, tends to keep 

 the uterine membrane in a condition of irritability which enables it 

 to respond very promptly to the stimulus brought to bear upon it by 

 the fertilized ovum. It is thus in the best possible condition to de- 

 velop the decidual membranes. 



CHAPTER XCV 

 THE DEVELOPMENT OF THE EMBRYO 



The Migration of the Ovum. — In those animals in which the ovary 

 is enveloped by a peritoneal pouch into which the Fallopian tube 

 opens, no special difficulty confronts us in explaining the migration of 

 the newly formed ovum into the uterus. In those aninials, on the 

 other hand, in which the ovary and fimbriated extremity of the Fal- 

 lopian tube are not in direct contact with one another, we are forced to 

 assume that the ovum first escapes into the peritoneal spaces and then 

 enters the tube from without. This manner of migration is exemplified 

 by the human female. Attention was first called to this possibility by 

 Bischoff,^ who found that animals possessing bifurcated or bicornuated 

 uteri frequently present corpora lutea in the ovary opposite to that 

 horn of the uterus in which the embryos are developing. Two explana- 

 tions may be offered for this occurrence, namely: (a) that the ovum has 

 penetrated the tube on the same side and has later on been forced into 

 the cornu uteri of the opposite side, and (h) that it has migrated to the 

 opposite side to begin with and has then entered the tube and uterine 

 horn of the same side. The former process is called internal migra- 



^ Marshall and Jolly, Phil. Transact., R. Soc, London, 1905. 



2 Ann. de gyn. et d'obst., 1909. 



3 Die Entwickelung des Kanincheneies, 1842; also Kussmaul, Von dem Mangel, 

 Verktimmerung und Uberwanderung des Eies, Wtirzburg, 1859. 



