8 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, i.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. EMBRYONIC PRIMORDIA AND THE LAW OF 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 earliest 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 



