64 THE DEVELOPMENT OF THE CHICK 
directed away from the observer to the right, when the broad 
end of the egg is to the observer’s left as in Fig. 32 A. 
The definiteness of orientation of the embryo with reference 
to the axis of the egg enables one to distinguish anterior and 
posterior ends of the blastoderm before there is any trace of an 
embryo; and while there is no possibility of orientation by 
examination of the blastoderm itself, or when such orientation is 
otherwise extremely difficult. By the method of orienting the 
blastoderm with reference to the axis of the shell, observers have 
been able to discover important features of the early development 
which would otherwise, no doubt, have escaped observation 
The relation is of interest in other respects discussed in their 
appropriate places. (See p. 15.) 
Chronology (Classification of Stages). The development of 
an animal is an absolutely continuous process, but for purposes 
of description it is necessary to fix certain stages for comparison 
with those that precede and those that follow. Each stage has 
a certain position in the continuous process, and the correct ar- 
rangement of stages is therefore a sine qua non for their correct 
interpretation. This may seem a very simple matter seeing that 
development is in general from the more simple to the more 
complex. And it would be so if it were not for the fact that 
embryonic stages, like the adult individuals of a species, vary 
more or less, so that no one embryo is ever exactly like another. 
These embryonic variations involve (1) the rate of development 
of the whole embryo, so that at a given time in the process no 
two embryos are in exactly the same stage; (2) the relative rates 
of development of different organs; (3) the size of the embryo, 
for embryos of the same stage of development may vary some- 
what in size. 
Although the total period of incubation is fairly constant in 
the hen’s egg, about twenty-one days, yet there is great variation 
in the grade of development of embryos of the same age, especially 
during the first week. This is due to two main factors: first, 
variation in the latent period, that is the time necessary to start 
the development of the cooled blastoderm after the egg is put 
into the incubator, and second, to variation in the temperature 
of incubation. Individual eggs may vary in rate of develop- 
ment when these two factors are constant, but this difference is 
relatively shght. Other things being equal, the latent period 
