16 CARL GOTTFRIED HARTMAN 



difference to be noted between the Australian and the American 

 species. Further details of the ripe ovum will be given in the 

 next section which deals with the freshly discharged eggs. 



b. The tubal ovum 



The eggs of the two lots (Nos. 56 and 76) removed from the 

 oviduct agree in all essential respects. They had apparently just 

 entered the oviduct, for only a mere trace of albumen had been 

 laid down on the surface of the eggs. The eggs are normally 

 shed simultaneously from a given ovary and both ovaries dis- 

 charge their eggs almost if not quite at the same time. In the 

 2ase of No. 76 the eggs from one ovary were a trifle late and were 

 lost in the body cavity, for, though discharged, they were not 

 to be found in the oviduct. 



The eggs discharged in the second or third oestrus period of 

 any given animal, however, show greater variation in size and 

 age. In one case an egg was found in the oviduct, while its 

 fellows had already been for some time in the uterus. The 

 ovaries of animals in the second oestrus usually contain some 

 large unruptured follicles along with fresh corporea lutea. Hencej 

 it is apparent that the discharge of eggs in the second oestrus is 

 not as regular as in the first. The opossum has only one sexual 

 season a year and normally (i.e., in the wild state) only a single 

 oestrus period. 



The tubal eggs are usually elliptical, rarely oval or spherical 

 in shape (figs. 1 and 3), as seen both in the fresh state and in 

 most sections. They measure on the slide about 0.137 by 0.115 

 mm., but range from 0.135 by 0.12 to 0.098 by 0.086 mm. The 

 eggs of batch No. 76 are a little the larger, measuring on the 

 average 0.146 by 0.123 mm. against 0.127 by 0.106 mm. for those 

 of batch No. 56. 



The egg is surrounded by a *rather well-defined homogeneous 

 membrane, the zona pellucida (fig. 3). In several cases this is 

 completely surrounded or only partly covered by a thin layer of 

 albumen laid down in several laminae. The zona can be easily 

 distinguished from the albumen on account of its dense, homo- 

 geneous structure. In some eggs the zona is not so well recog- 



