S THE DEVELOPMENT OF THE CHICK 
to push the analysis of the subject farther, and to furnish the 
true interpretation of the observations. In some cases experi- 
ments have confirmed the physiological deductions of pure ob- 
servation, and in many cases have decided between conflicting 
views. Not all embryological experiments, however, are essays 
in the direction of a physiology of development; some are directed 
to the solution of morphological problems, as, for instance, the 
origin of the sheath cells of nerves, or the order of origin of so- 
mites, or the relation of the primitive streak to the embryo. 
Experimental embryology is, therefore, not synonymous with 
physiology of development. 
Physiology of development must proceed from an investiga- 
tion of the composition and properties of the germ-cells. It 
must investigate the role of cell-division in development, the 
factors that determine the location, origin, and properties of the 
primordia of organs, the laws that determine unequal growth, 
the conditions that determine the direction of differentiation, 
the influence of extraorganic conditions on the formation of the 
embryo, and the effects of the intraorganic environment, 7.e., 
of component parts of the embryo on other parts (correlative 
differentiation). Each of these divisions of the subject includes 
numerous problems, which have attracted many investigators, 
so that the materials for a consistent exposition of the physiology 
of embryonic development are being rapidly accumulated. This 
direction of investigation is, however, one of the youngest of 
the biological disciplines. It will be seen how far it is removed 
from attempts to explain embryonic development by a single 
principle. 
IV. Empryonic PRIMORDIA AND THE Law or GENETIC RE- 
STRICTION 
In the course of development the most general features of 
organization arise first, and those that are successively less general 
in the order of their specialization. For every structure, there- 
fore, there is a period of emergence from something more general. 
The earhest discernible germ of any part or organ may be called 
its primordium. In this sense the ovum is the primordium of 
the individual, the ectoderm the primordium of all ectodermal 
structures, the medullary plate the primordium of the central 
and part of the peripheral nervous system, the first thickening 
