348 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921 



His studies on the embryology of this form were begun eight or nine 

 years ago at the instance of Prof. J. T. Patterson, following the 

 appearance of Prof. J. P. Hill's beautiful monograph on the early 

 stages of Dasyurus, the Australian "native cat." Notes were taken 

 for various purposes on 685 female opossums and a complete series 

 of several thousand eggs, embryos, and pouch young was collected. 

 The work on so large a scale was made possible through liberal 

 grants from the Wistar Institute of Anatomy and Biology (Phila- 

 delphia) and a $500 fund given by Mr. H. A. YVroe, of Austin, Tex., 

 in 1920. Although the original interest centered around the develop- 

 ment of the embryo, observations and experiments were made upon 

 the physiology of reproduction. This paper will deal with both 

 topics. 



SPECIES STUDIED 



Almost all of the experimental animals belonged to the species 

 Didelphys virginiana, of which there is a gray and a black phase. 

 These apparently occur in the proportion of about 10 to 1, respec- 

 tively. The color depends entirely upon the color of the overhair, 

 which is white in the gray phase, black in the black phase. Three 

 albino and three cinnamon mutations have also been acquired, the 

 latter a gift from Dr. Charles McNeil, of Sedalia, Mo., where the 

 brown form is said to occur rather often. It would seem that the 

 same factors underlying hair color are widespread throughout the 

 mammalian series, including the marsupials. 



THE FEMALE REPRODUCTIVE ORGANS. 



Before going on to the main subject it is essential that the reader 

 be familiar with the anatomy of the reproductive organs of the 

 female opossum. The generic name Didelphys, given to the opos- 

 sum by Linnaeus, has reference to a distinctive feature of marsupials, 

 namely, the presence of two separate and distinct uteri (fig. 5, pi. 2; 

 and pi. 3) ; for in the higher mammals there is but one, which may. 

 however, be divided into the central " body " and the two " horns." 

 Each uterus opens by a separate " os." not directly into the vagina 

 but into a lateral vaginal canal; and the two lateral canals in turn, 

 after describing a loop, empty separately into the median vaginal 

 canal or vagina proper. This opens with the rectum into a short 

 " cloaca," so that there is in marsupials but one external opening 

 ordinarily visible posteriorly, namely, the cloacal orifice. It is thus 

 seen that in marsupials the paired arrangement of organs obtains 

 not only for ovaries and Fallopian tubes, as in all mammals, but 

 also involves the uteri and the lateral vaginal canals. From the 

 standpoint of their physiological behavior the lateral vaginal canals 



