SUMMARY OF CURUENT RESEARCHES. 567 



Ovulation marks the end of the postoestrus, and occurs generally 

 about five or six days after oestrus. It is spontaneous and independent 

 of copulation, It is remarkable because of the large number of ova 

 liberated. Ovulation is succeeded by pregnancy or by pseudo -pregnancy. 



Fertilization is effected in the upper part of the Fallopian tube, 

 and the second polar body is there given off. As a rule, more young 

 are born than can possibly survive, OAving to the limited accommodation 

 in the pouch. The gestation period is not less than eight and not 

 more than fourteen days, but the interval between copulation and birth 

 is usually considerably longer. Corpora lutea are formed, and persist 

 throughout the greater part of the time that the animal is lactating. 

 The nursing period includes seven to eight weeks of fixation to the 

 teat and eight to nine weeks of freedom in the pouch, but dependence 

 on the mother for food. After this time the various organs gradually 

 return to a state of rest. 



In pseudo-pregnancy the ova fail to develop, but there ensues a 

 series of changes in the reproductive organs essentially identical with 

 those met with in pregnant females. Corpora lutea are formed, the 

 pouch enlarges, its glands hypertrophy, the mammary glands enlarge, 

 the uteri enlarge and become vascular, the uterine mucosa passes through 

 a series of changes, progressive, regressive, and regenerative. In the 

 indefinite period of metoestrus, all the reproductive organs return to a 

 state of rest. 



In the Marsupial, ovulation only occurs at an interval of some days 

 after oestrus ; in the Eutherian, ovulation either coincides with or follows 

 immediately after oestrus. There is a post-oestrus period in Marsupials. 

 In the Marsupial the degenerative changes in the uterine mucosa succeed 

 ovulation ; in the Eutherian they not only precede ovulation but also 

 oestrus. This is the most striking difference in the two cycles. 



There can be little doubt that the uterine degenerative changes 

 seen during the pseudo-pregnancy period in the Marsupial are equiva- 

 lent to those which take place in the Eutherian uterus during prooestrus. 

 These prooestral changes in the Eutherian uterus condition the appear- 

 ance in some members of the sub class (e.g. Primates) of a sanguineous 

 discharge — the menstrual flow. This is regarded by the authors as the 

 morphological and physiological equivalent of the degenerated epithelial 

 elements and blood extravasations met with in the pseudo-pregnant 

 uterus of the Marsupial. They uphold the view that menstruation is 

 associated with preparations for the reception of a fertilized ovum, or 

 of degeneration of the mucous membrane which through failure of 

 pregnancy has not been able to fulfil its purpose. They do not agree 

 with those who regard menstruation as identical with heat, or who 

 regard it as the physiological equivalent of the prooestrum. The authors 

 regard the monoestrous condition as the primitive one, and the poly- 

 oestrous as a secondary improvement. 



Placenta of Dasypus Novemcinctus.* — H. Strahl describes the 

 peculiar structure of this placenta, which is unlike any other. There is 

 a large inter- villus space, traversed by pillars of uterine mucous-membrane 



* Anat. Anzeig., xliv. (1913) pp. 440-7 (3 figs.) 



