BREEDING HABITS OF THE OPOSSUM HARTMAN. 361 



5 days old from another litter; these are seen in the lower part of 

 the figure (at c). At the upper margin of the figure (at a) is shown 

 a section of the mammary gland at parturition. Three tiny teats 

 may be seen ; these have been drawn out to double their length by an 

 hour's traction upon them by the thick tongue of the young. As the 

 pouch young grows the mammary gland thickens and the teats 

 lengthen greatly (figs. 47 and 45, pi. 10). Thus the young re- 

 mains attached within the pouch for about 70 days before relinquish- 

 ing its hold. And for another 30 clays it moves freely about the 

 mother clinging to her hair or (rarely) holding on to her tail with 

 their own and entering the pouch for food or, when startled, for 

 protection. 



There is another point of interest in connection with the birth of 

 marsupials, which was discovered by Professor Hill in the case of 

 two Australian and one South American species. Professor Hill 

 found that the embryos at birth do not pass out by the circuitous 

 route through the lateral vaginal canals but are forced out by a new 

 and purely temporary passage (the " pseudo-vaginal passage ") lead- 

 ing from the uteri straight to the median vaginal canal. This phe- 

 nomenon the writer also observed in the opossum. As Professor 

 Hill suggests in a personal letter : " It seems probable that this 

 passage occurs in all marsupials and is a class character." 



It is thus seen that the popular notions concerning the generation 

 of the opossum have no foundation in fact. Marsupials are indeed 

 unique in the extremely early birth and the performance of the 

 newly born young which is little less than marvelous; but other- 

 wise there is no fundamental difference between the higher and the 

 lower animals. 



EXPLANATION OF PLATES. 



(Those marked with an asterisk (*) were photographed in the living state; that is, 



immediately on removal from animal.) 



Plate 1. 



* Fig. 1. Gray male and albino female of the opossum, Didelphys virginiana. 

 *Fig. 2. At home on the Red River (Photo by Dr. E. H. Sellards). 



* Fig. 3. Thoroughly " playing 'possum," a rare occurrence. 

 Fig. 4. Pouch young attached to teats. XI. 



Plate 2. 



* Fig. 5. Generative organs and bladder, at resting stage between two cestrous 



periods. The two ovaries (ov.) the two narrow uteri (ut.) con- 

 nected with the broad ligament, the two lateral vaginal canals 

 (1. v. c.) and the median vagina (vag.) are well shown. XI. 



