26: CARL G. HARTMAN 
But the ages given for all his early stages are far too low, because 
the author greatly overestimated the postoestrous period, that 
is the interval between copulation and ovulation, which he 
states to be five days. The time of beginning of cleavage he 
fixes at ‘exactly five times 24 hours,’ a period which he ap- 
parently determines on the basis of one experiment in which he 
secured what I regard as fragmenting eggs in a condition that 
accords very well with eggs about three days old. Again, his 
10-hour vesicle is nearer three days old and his 32-hour vesicle 
nearer four days; hence the interval of twenty-two hours between 
these two stages is substantially correct. In a subsequent paper 
I shall discuss these time relations from the abundant, though by 
no means simple and harmonious data on hand. 
c. Material and technique 
1. Material. The opossum eggs on which the present study is 
based represents collections made during four seasons. In 1914 
- eighteen litters or batches of fertile eggs were secured; in 1915, 
seventeen litters; in 1916, fifteen litters, and in 1917, 37 litters— 
a total of 87 litters. These refer, of course, only to stages 
coming within the field of this paper, for besides these many 
litters of older stages were collected; and unfertilized eggs were 
removed ad nauseam. The 87 fertile litters, which include eggs 
through the bilaminar stage, contained 1009 eggs, of which 641, 
or nearly two-thirds, are normal. Thus, about one-third of the 
eggs secured from pregnant females are unfertilized or abnormal, 
chiefly the former. My previous estimate of one-sixth is there- 
fore too low. The average number per litter is 11.5, the ex- 
tremes are 1 (No. 94) and 22 (No. 346’), not taking into con- 
sideration No. 117’, which numbered 43 eggs by virtue of the 
compensatory hypertrophy of the ovary. Table 1 summarizes 
the number of eggs mentioned under ‘‘ History of the Animals’’ 
in the next section. 
2. Animals used; reference to illustrations. In the following 
summary a brief protocol is presented of each animal furnishing 
eggs used in the present study. The data for Nos. 21 to 144 
