DEVELOPMENT OF THE OPOSSUM 19 



('87) states: ''Der Eintritt der Spermatozoen in den Perivitellin- 

 raum geschieht im oberen Theile des Oviducts, wie die Anwesen- 

 heit derselben in den auf dieser Strecke vorgefundensen Eiern 

 geniigend beweist."^ But it will be observed that the actual 

 entrance of the spermatozoon was not seen by Selenka, nor does 

 he anywhere mention the pronuclei. 



The ovum becomes surrounded with an albumen layer and a 

 shell on its descent down the oviduct (fig. 5, 3). The albumen 

 is laid down in successive delicate lamellae, between which 

 numerous spermatozoa are seen in some of the eggs ; or epithelial 

 cells from the oviduct may become included in the albumen. 

 One case was noted in which the spherical mass of epithelial cells 

 instead of an ovum became surrounded by albumen and shell. 

 In another case an ovum and a mass of cells were both included, 

 and in a third case, two eggs, each separately provided with some 

 albumen, were enclosed together in a shell. 



c. Appearance of young uterine eggs 



The eggs of female No. 58 are, to all appearances, the youngest 

 found in the uterus. They are all unfertilized but otherwise are 

 normal and show little or no signs of disintegration. Since the 

 egg protoplasm very rapidly breaks up into fragments of various 

 sizes, we may assume that the eggs of this lot had just arrived 

 in the uterus. The shell membrane is, moreover, only 0.001 

 mm. in thickness and is so delicate as to shrink into many folds 

 which lie, in the fixed specimens, snugly against the sphere of 

 coagulated albumen. The shell grows in thickness in the uterus, 

 hence the thickness of the shell is roughly a criterion as to the 

 age of the eggs. But this is true only in very general terms, for 

 there is no uniformity of thickness of shell among different 

 bartches of eggs in the same cleavage state (see Summary of this 

 paper). The eggs of various individuals may, of course, pass 

 through the Fallopian tube at different rates, hence the eggs 

 passing the more slowly would be provided with thicker shells 

 on entering the uterus. 



« Selenka ('87), p. 110. 



