INTERPRETATION OF THE EARLY STAGES. 2IQ 



a graded, tri-axial series of unlike environments, and three similar graded series of 

 cells, unlike as to age is established. This dual series of unlike conditions, and 

 cells of unlike lineage, coincides, as nearly as may be determined, with a third and 

 fourth series, namely the lines of morphological and physiological differentiation; 

 hence, the conclusion is justified that the two latter are the formal, or kinetic ex- 

 pression of the two former. That is, metamerism, or the succession of unlike 

 parts in a cephalo-caudal direction, bilateral symmetry, or the succession of 

 unlike parts in a bilateral direction, and the formation of superimposed germ layers, 

 are the inevitable results of the locally diverse physical and chemical conditions, 

 and the locally diverse cell lineages created by apical growth. They cannot 

 therefore be the result of the unfolding, or distribution, of diverse specific forma- 

 tive materials. 



It would therefore appear that there is a definite order in which various tissues 

 are automatically created by their individual environments, the degree of histo- 

 logical specialization having a constant time and space relation to the germinal 

 axis. That is, if we assume that the nervous tissue is the most highly specialized, 

 then it is clear that at any period of development, the most highly specialized tis- 

 sues predominate in the germinal axis, and that: a. the grade of specialization 

 diminishes from any point in that axis right and left to the germinal margin, or 

 to the periphery of each half metamere; and b. the grade of development of each 

 member of the half metamere reaches its maximum at some point behind the 

 cephalic end of its series, and gradually diminishes toward the germinal apex 

 at the caudal end of the body. (Fig. 157.) 



It will also be observed that in passing from the outside of the sphere inward, 

 the degree of morphological and physiological complexity in the four superimposed 

 layers varies inversely as the distance from the germinal axis. 



Conclusion. Continuous aggregation of like materials is an impossibility, 

 because all growth, or aggregation of materials, automatically creates for each 

 of the constituent parts unlike time and space conditions, which in turn control 

 further growth and differentiation. The unlike conditions thus created, tend to 

 form parallel, or concentric, isogeminal and isomorphic shells and zones, the re- 

 sulting form, mode of growth, and action of the constituent parts being the visible 

 expression of the local conditions created by growth. 



Natural selection, external environment, and heredity play no part in the 

 creation of the physical and chemical framework of living things. 



II. MORPHOLOGICAL INTERPRETATION OF THE EARLY STAGES OF EMBRYONIC 



GROWTH. 



All metazoa may be reduced to one of two types of structure, the radiate 

 and the bilaterally symmetrical. These two types have not arisen independently 

 of each other; they are genetically related, the latter being derived from the former. 

 The bilateral type may become secondarily asymmetrical (certain molluscs and 

 arthropods), or, by the complete suppression of one side, it may develop into a 



