DEVELOPMENT OF THE OPOSSUM 33 
batch is 0.142 mm., the maximum width 0.120 mm. The 
average of eight eggs of batch No. 76 is 0.134 x 0.113 (av. 0.123) 
mm.; the maximum length is 0.148 and maximum width 0.125 
mm. But this batch includes also two very small eggs measur- 
ing 0.100 x 0.085 (av. 0.093) mm. and 0.090 x 0.083 (av. 0.087) 
mm., the latter being the smallest egg in the collection. Three 
eggs of batch No. 307 average 0.126 x 0.116 (av. 0.121) mm. in 
the preparations; the size of the fresh eggs of this batch, shown 
in figure 7, plate 1, cannot be given because the magnification 
of the photograph is not known. The two living eggs of batch 
No. 313 shown in figure 1, plate 3 measure 0.119 x 0.105 (av. 
0.112) mm. and 0.106, respectively; the average of eleven eggs 
of batch No. 313’, as photographed in the living state, is 0.108 x 
0.099 (av. 0.103). In the preparations, three eggs of batch 
No. 313 measure 0.105 x 0.091 (av. 0.098) mm., indicating some 
shrinkage of the eggs in the histological processes. Batch 
No. 351’ (fig. 2, pl. 3) average 0.110 x 0.096 (av. 0.104) mm. in 
the living state; three preparations from this batch measure 15 
per cent less, namely, 0.102 x 0.076 (av. 0.089). 
3. The distribution of yolk. The opossum egg, in common 
with the eggs of other marsupials, is rich in yolk or other lipoid 
deposit, which partly accounts for their larger size (figures on 
pl. 14). The fat occurs in the form of granules or spherules, 
many or perhaps all of which stain black with osmic acid. Eggs 
fixed in Bouin’s fluid show numerous vacuoles from which the 
fat is dissolved in clearing. The fat content of the eggs renders 
them much less transparent; but in the living state the globules 
may be seen in the egg and they also appear distinct near their 
outer limits of distribution in the photographic negatives. taken 
by transmitted light in salt solution. Thus the negative from 
which figure 1, plate 3, was made shows in detail oil globules - 
quite similar in distribution to those shown in figures 12 and 18, 
plate 14. A study of the fresh eggs and photographs of them 
convinces me that the fixed and sectioned specimens accurately 
show the true details of these eggs, little altered by the histo- 
logical processes. 
