GROWTH AND MATURITY OF SALMON IN THE OCEAN 
21 
readjust the eggs, and repeat the operation. The measurement of 10 eggs by this 
scale gives, by simply moving the decimal point one place to the left, the average 
size of the eggs to hundredths of a millimeter, a degree of accuracy which is un- 
necessary in the great majority of cases. In preparing eggs for this measurement 
it is necessary to free them very carefully from the ovarian membranes, so as not 
to break the delicate egg membrane and yet to clear them of all shreds of tissue 
which might tend to affect the measurement. The smaller eggs — those less than 
1 mm. in diameter — were measured by means of a microscope fitted with an eye- 
piece micrometer, carefully calibrated against a stage micrometer. In using this 
method it was necessary, of course, to measure the 10 eggs separately, and then the 
average of these measurements was found. 
If one examines the eggs of chinook salmon taken in fresh water during their 
spawning migration, it is found that the eggs of different individuals vary only 
slightly in size. A similar examination of the eggs of females taken in the ocean 
discloses that there are wide variations in size. A group with eggs appro^dmately 
the same size as those of the fish taken at the same time of year in the river can 
readily be selected. In addition to this group, however, many of the females 
have eggs distinctly smaller than any found among the spawning run in the river, 
and it is possible many times to separate these smaller eggs into two or more groups, 
even without careful measurement. When this observation was first made it seemed 
probable that each size group of eggs indicated a different degree of maturity, 
so that an analysis of the relative maturity of the fish in a given catch could be 
accomplished merely from an examination of a series of the ovaries. The assump- 
tion was that the fish with the largest eggs would mature and spawn during the 
year in which they were captured, that those with eggs falling within the next 
smaller group would mature during the next year, and so on. It has been found, 
on closer analysis, that while the size of the eggs alone forms a very satisfactory 
diagnostic character for distinguishing between fish that will mature during the year 
in which they are captured and those that will not mature for at least one more 
year, it can not be depended upon to distinguish between those that will mature 
in one year from those that will mature in two or more years. It has been found 
that the eggs grow in proportion to the rest of the fish until, approximately, the 
beginning of the growing season, which is destined to end in the maturing and 
spawning of the individual. With the onset of this last growing period, however, 
the rate of growth of the eggs is relatively accelerated and a differential growth 
sets in, so that the eggs gradually increase in size relative to the size of the fish. 
As a result, the eggs of the maturing fish are relatively larger than those of the 
immature specimens. This fact makes it possible to distinguish between those 
individuals that are maturing and those that will not mature for at least one year. 
The observed differences in the size of the eggs of the immature fish taken in 
the ocean are due only to corresponding differences in the size of the fish. As will 
be shown later, there is a high degree of correlation between the size of the eggs 
and the size of the fish. The observation that the eggs may be grouped on the basis 
of size is dependent upon the fact that the differences in size between the younger 
age groups of chinook salmon, as in many other animals, is often so marked that 
