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 slight. Other things being equal, the latent period 



