WHITE AND YELLOW YOLK OF OVA A461 
of the yolk necessary to make it capable of producing an animal. 
Nevertheless five to eight days suffice to supply the missing 
ninety-nine parts. 
THE THICKNESS OF THE STRATA OF WHITE AND YELLOW YOLK 
OF THE COMMON FOWL 
The measurement of the thickness of a layer of yolk offers some 
difficulties and. can rarely be done directly on a single layer; the 
reasons being that one stratum merges very gradually into 
another and that the strata are often very indistinct. More 
frequently, though by no means in every egg, a series of well- 
marked layers can be found and a measurement made over all; 
the number of strata—or rather of pairs of strata—may be 
easily counted. When the total measurement is divided by this 
number one obtains the thickness of a combined layer of white 
and yellow yolk. 
The result of eight such measurements is recorded in section 
B of table 1. These are typical of nearly forty reliable measure- 
ments, and indicate a thickness of about 2.0 mm. (1.4 — 2.5) 
for a layer of white and yellow yolk combined. 
The layers of yolk can sometimes be seen in the fresh eggs, 
proving that they are not artifacts; but for the purpose of measure- 
ment it is usually best to hard-boil them, and section (under 
water) from one side until the exact ceuter of the egg is reached. 
Sometimes it will be found advantageous to put the egg thus pre- 
pared in weak iodine solution for a time. This treatment seems 
occasionally, though not always, to strengthen the contrast 
between the layers of white yolk and those of the yellow variety. 
For reasons stated above it is impossible satisfactorily to measure 
the thickness of a layer of white yolk. It can be said with con- 
fidence, however, that this so-called layer has but a fraction of 
the thickness of the adjoined yellow layer. Perhaps one errs but 
little in saying that the former usually has from one-fourth to 
one-eighth the thickness of the latter. 
