496 EMBRYOLOGY OF THE TURTLE. PART sie 
work undoubtedly still contains, I feel the more the responsibility I have assumed, 
But, if 
I cannot expect to exhaust the subject, I may at least hope to show how 
in undertaking to write anew the Embryology of that order of Reptiles. 
instructive this field may become for the American student, and how important 
it is for science in general. Every European embryologist must envy the oppor- 
tunities our naturalists have in this respect; and it is the duty of those who pos- 
sess such advantages to supply fully and freely any additional information which 
a thorough comparison of the structure and embryology of the different genera 
and families of our Turtles may afford, and which is not already included in the 
following pages. 
The age at which Turtles begin to lay has been ascertained, with sufficient 
precision, only for one species, our common Chrysemys picta. By the help of a 
series of specimens, from those just born up to adult ones, it was possible to trace 
the progress of growth of the ovarian eggs till they were ready to drop into 
the oviduct; and thus the fact was elicited, that the eggs do not begin to differ in 
size among each other by any readily appreciable amount until the seventh year, and 
that the process of reproduction by laying is not commenced before the eleventh year. 
Several other genera of this and other families were examined in reference to this 
point, but for want of materials the investigation was not carried on so exten- 
sively nor with so much precision as with Chrysemys picta; yet enough has been 
seen to warrant the assumption, that from the eleventh to the fourteenth year? 
is about the age at which most, if not all, our native fresh-water Turtles lay 
their eggs for the first time. 
Again: the time of the year at which they lay is the same for both the 
northern and the southern species, without reference to physical differences, such 
as temperature, moisture, etc. or climate in general. Graptemys LeSueurii, which 
lays as early as the first of June, gives the earliest instance of incubation in the 
year, and this is a western and south-western species. Chelydra serpentina, the spe- 
cies most widely distributed in the United States, at the North lays as early as 
the tenth of June, and continues to do so till the twenty-fifth: some individuals dis- 
posing of their burden as early as the first date, and others as late as the latter. 
1 A careful comparison of the relative distance of 
the successive lines of growth of the scales may sat- 
isfy any one that the Turtles grow more rapidly 
during the first ten or twelve years of their life ; and 
that after the twelfth or fourteenth year the rate of 
increase is considerably diminished. From the facts 
observed in our little Chrysemys picta it is certain 
that this is also the period at which they begin to lay. 
There exists, no doubt, some difference between differ- 
ent families; but, judging from the change in the rate 
of increase after the twelfth or fourteenth year in 
different species, there can be no doubt that this is a 
eritical period in the life of all the scaly fresh-water 
and land Testudinata, and Chrysemys picta shows 
that this is connected with the period of their first re- 
production. 
