58 CARL GOTTFRIED HARTMAN 



GENERAL SUMMARY 



1) Maturation of the opossum egg begins in the ovary, where, 

 as in other mammals, the first polar body is given off (fig. 2, 

 pages 15-16). 



2) An average of twenty-two eggs are shed at one ovulation, 

 but the number may rise to forty-five. This overproduction of 

 eggs is accompanied by a high percentage of mortality at each 

 stage in the development of the opossum, for unfertilized or 

 otherwise abnormal eggs are found in each lot and dead foetuses 

 are of frequent occurrence. On removal of the ovary and uterus 

 from one side at the time of the first ovulation, the remaining 

 ovary may compensatorily double the normal production of eggs 

 at the second ovulation (rages 10-15). 



3) Measurements of a dozen full and unshrunken ovarian ova 

 gave an average of 0.165 x 0.135 mm. Unsegmented tubal and 

 uterine eggs in the sectioned and stained preparations measure 

 less, namely from 0.146x0.123 mm. to 0.095x0.080 mm., or 

 an average from six lots of 0.111 x 0.107 mm. Hence the eggs 

 of the opossum are on the average larger than most mammalian 

 eggs. They are much smaller than the eggs of Dasyurus, which 

 measure 0.27x0.24 mm. (fig. 4 and its legend). 



4) The ripe egg of the opossum exhibits no evidence of polarity 

 except that afforded by the position of the polar body and chromo- 

 somes of the egg nucleus. No polar concentration of deuto- 

 plasmic substance is ever encountered in the unsegmented opos- 

 sum egg. The yolk granules or yolk spherules are for the most 

 part situated in a peripheral or submarginal region, just beneath 

 a thin yolk-free layer of granular cytoplasm (figs. 1, 3, 6, 8; 

 pages 15-18). 



5) Insemination occurs in the oviduct, where doubtless the 

 second polar body is given off; neither process was, however, 

 actually observed. After insemination the egg receives, through 

 the secretive activity of the gland cells lining the o^dduct, the 

 secondary egg envelopes: — albumen and shell membrane. The 

 albumen is homologous with the albumen covering of other 

 Amniota; the shell membrane is homologous with the shell of 

 Monotremata (figs. 5 and 6, pages 18-20). 



