DEVELOPMENT OF THE OPOSSUM 



59 



6) When the eggs pass from the oviduct the shell membrane 

 is thin and delicate, measuring 0.0012 mm. in thickness. It 

 grows thicker rapidly, soon attaining a maximum which remains 

 practically constant until the didermic blastocyst is fully formed. 

 There is no progressive growth in thickness of shell membrane 

 as in Dasyurus. That different batches of eggs, even in the 

 same stage of advancement, vary considerably in this respect 

 may be seen from the following table (figs. 4 and 5, page 19). 



7) Uterine eggs in early cleavage measure from 0.4 to 0.5 mm. 

 through the shell membrane, or an average of about 0.45 mm. 

 Hence the albumen layer is on the average 0.15 mm. in thickness. 

 This thickness is maintained until the fully formed blastocyst 

 begins to grow at the expense of the albumen. The egg grows 

 very little in diameter of shell, being only 0.6 mm. in diameter 

 at the time entoderm formation sets in or 0.7 to 0.9 mm. at 

 the completion of the didermic blastocyst, when nearly all the 

 albumen has been digested (fig. 4, pages 20 and 56). 



8) The pronuclei are fully formed when the egg enters the 

 uterus. At first they occupy a position within a yolk-free area 

 at one pole, but soon migrate to the center, where the first 

 cleavage spindle takes its position (figs. 4, 6 to 8, pages 21-22). 



