SUPKlihWrA TJo.y. 1 7 5 



larger domesticated animals. The cases recorded have not been 

 sutliciently investigated to convince those who deny the likelihood of 

 two conceptions taking place after a certain interval ; and it must be 

 admitted that a true explanation of such a singular occurrence has not 

 yet been offered. A lapse of time occurring between the birth of two 

 animals is no strong proof of a second impregnation during conception ; 

 for, as we shall see hereafter, when twins are conceived from the same 

 intercourse, it may and does happen that one ovum does not attain 

 maturity so soon as the other, and is eitlier rejected or retained after a 

 more or less lengthened interval — a circumstance wliich might mislead. 

 And again, with regard to the size of twins, it is not at all unusual to 

 find one larger and more developed than the other, though both were 

 produced at the same period. Putting aside the question of superfuita- 

 tion in animals, the anatomical disposition of whose generative organs 

 evidently permits such an occurrence, a little consideration will show 

 tliat usually there are physical obstacles which would appear to offer an 

 insurmountable barrier to a second impregnation, after conception has 

 been achieved for a short time. Soon after that event has taken place, 

 the entrance to the ' uterus is closed by the shut sac enveloping the 

 embryo, and which adheres closely to the inner surface of the organ 

 throughout its entire extent — covering the orifices of the os uteri and 

 Fallopian tubes. In addition to tiiis, the canal of the cervix is during 

 gestation rendered still more impervious by the thick viscid mucus 

 secreted by its glands. Such being the case, it w'ill be obvious that a 

 second impregnation cannot occur, if it be necessary for this purpose 

 that the spermatozoa pass into the uterus, or even to the ovaries ; for 

 the whole is hermetically sealed after a certain time. For a second 

 impregnation to occur during conception, fecundation must take 

 place before this closing-up of the uterus and Fallopian tubes — an 

 interval too brief after the primary impregnation to make much 

 difference in the respective developments of the young animals. In 

 cases in which there is a double uterus, or in which conception occurs 

 in only one horn, superfcetation is possible, and one parturition may 

 not be followed by another for some consideral)le time. However this 

 may be, there are certain facts recorded whicli need explanation if super- 

 fcetation be not admitted as possible. 



SECTION 11.— EXTRAUTERINE PKKCNANCV. 



In studying the development and progress of the ovum, after its 

 escape from the ovary and impregnation by the spermatozoa of the male, 

 we saw that a peculiar arrangement existed in the presence of the 

 fringed border at the extremity of the Fallopian tube, which grasped 

 the ovum and permitted it to be conveyed into the canal on its way to 

 the uterus. From certain causes which are not yet clearly understood, 

 it sometimes chances that the ovum, instead of taking this its normal 

 course, either remains in the ovary, is arrested in its progress tlirough 

 the tube, or, escaping the fimbriated extremity of the latter, falls into 

 the peritoneal cavity, or glides between the folds of peritoneum con- 

 stituting the broad ligament, or between the serous and mucous mem- 

 brane of the uterus ; in all of which situations nature makes an effort to 

 afford space and nutrition for the embryo, and thus supply the place of 

 the uterus. This effort, however, as might be anticipated, is only par- 

 tially successful, and after attaining a more or less imperfect develop- 

 ment, the futus perishes from lack of nourishment. 



