460 EMBRYOLOGY OF THE TURTLE. Part JIL. 
9,) the light ring around the germinal vesicle becomes still lighter, and the dark 
spot more sharply defined, until, when full-grown, (Pl. 9, fig. 10, and Pl. 9a, fig. 
32, 32a,) the yolk is orange yellow, the ring around the germinal vesicle dead 
white, and the spot above it a neatly bounded circular area, (Pl. 9a, fig. 32a,) 
resembling a pinhole over a dark background. 
It is important to notice, in this connection, that there is a marked difference 
in the gradation and relative size of the smaller eggs when compared to the 
larger ones. The innumerable minute eggs which are buried in the folds of the 
ovary exhibit, up to a certain size, every possible degree of development, from 
the smallest granule-like egg cells to characteristic eggs visible to the naked eye. 
There are immense numbers of these small eggs of every size, apparently in the 
same state of progress; and they seem all to form but one series, in which every 
successive stage is represented by an indefinite number of eggs. Not so with 
the larger eggs, from the time they exceed the size of a large pin’s head up to 
their full maturity. These larger eggs appear always in regular sets of a defi- 
nite number, and, what is particularly important, this number coincides with the 
number of eggs the different species of Turtles lay at one time. In Nanemys 
guttata, which lays two or three eggs, each set contaims only two or three eggs; 
in Chrysemys picta, which lays from five to seven egys, each set contains from 
five to seven eggs; and so with every species, even with those which, like Che- 
lydra serpentina, lay more than thirty eggs. Four such sets can readily be dis- 
tinguished in every ovary, one of which contains mature eggs (Pl 9, fig. 10) ; 
another set contains eggs about half that size (Pl. 9, fig. 8); a third set contains 
still smaller eggs, (Pl. 9, fig. 5, 6,) the size of which stands in the same relation to 
the second set, as those of the second to the first; the fourth is smaller still, in the 
same ratio (Pl. 9, fig. 1, 2, 3). Below these it is difficult to distinguish the different 
sizes, and impossible to determine which are the eggs likely to start in advance of 
the others, after the largest set has been laid. But the uniformity of the eggs of 
each set, the conformity of their number with that of the eggs laid by different 
Turtles, and the absence of eggs of intermediate sizes between those of different sets, 
can leave no doubt, that, after a certain time, the eggs of each successive brood are 
determined in the ovary, and undergo a long development, equal in duration to 
four times the interval which intervenes between the successive periods of laying. 
As I have satisfactory evidence that our Turtles lay only once a year, it follows, 
therefore, that an egg requires four years, from the time there exists a marked 
difference among the eggs of different sizes, to acquire its full maturity; not to 
speak of the length of time required for its formation and earlier development. 
We shall have occasion hereafter to consider the importance of these facts, in con- 
nection with the act of fecundation of the eggs. 
