OUTLINE OF DEVELOPMENT, CHRONOLOGY 67 



tions in the time of development in the oviduct and uterus, or 

 to slow development before incubation in warm weather, or to 

 individual variation. It becomes necessary, therefore, to find 

 some other system. The method followed by a considerable 

 number of investigators, namely to classify by the number of 

 somites, has been found to be best between about the twentieth 

 and ninety-sixth hours of incubation. In the table which follows, 

 therefore, this method of classification is used. For the sake 

 of brevity throughout the book a stage reckoned by the number 

 of somites will be written 1 s, 2 s, 3 s, etc. It is true that the rela- 

 tive rate of the development of organs varies slightly. Never- 

 theless, classification by number of somites is unquestionably 

 the most exact method up to the end of the fourth day at least. 

 Beyond this stage the method is difficult to apply, and after 

 about the sixth day the number of somites becomes constant. 

 After the fourth day the time of incubation is usually a suffi- 

 ciently exact criterion for most purposes: the latent period has 

 become a relatively inconsiderable fraction of the whole time 

 of incubation, and the embryos that survive, assuming fresh eggs 

 and normal temperature of incubation, are in about the same 

 stage of development. 



Classification of embryos by length is a favorite method 

 particularly in Germany, and it offers many advantages in the 

 case of some animals; under many conditions it is the only avail- 

 able method. But it offers considerable difficulties, the most seri- 

 ous of which come from the varying degrees of curvature of the 

 embryo. In early stages of the chick, for instance, up to about 

 12 s, the total length of the embryonic axis may be measured, 

 for the embryo is approximately straight. The cranial flexure 

 then begins to appear, and slowly increases to a right angle; 

 during this period there may be an actual reduction in length 

 of the embryo (cf. table, 14-16 s). Conditions are also compli- 

 cated by the fact that the head of the embryo is turning on its 

 left side at the same. time. The cervical flexure then appears 

 and causes a second reduction of the total length (cf. table 29- 

 32s). Later still the curvature of the trunk and particularly 

 of the tail develops in somewhat varying degrees and makes 

 bad matters worse. After these flexures are formed, let us say 

 at about eighty hours in the chick, it is customary to take the 

 so-called neck-tail measurement, that is, from the cervical flexure 



