2l6 EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



that under certain fixed uniform conditions protoplasm, or some of its constitu- 

 ents, has the property of growing, that is of producing more material like itself; 

 the growth taking place in the three planes of space at a uniform rate and to an 

 unlimited extent. If any deviations from these results occur, it must be due to 

 new conditions that arise either outside the growing mass, or which are locally 

 created within the mass by growth itself. 



It is evident that a single cell, or a group of like cells, endowed with this 

 initial power of self increase, in the very act of growing necessarily creates internal, 

 locally diverse physical and chemical conditions; and that these unlike conditions 

 will be arranged in regular graded zones. As it appears that these zones of unlike 

 conditions, in the main, coincide with the zones of histological and morphological 

 differentiation, they may be fairly assumed to be the principal causes of that 

 differentiation. That is, it appears that progressive differential growth is self- 

 creating, and that homogeneous, or undifferentiating growth is an impossibility. 



The details in the end results may be colored or modified by the presence 

 at the outset of foreign materials, or by changes in the external environment, 

 but they appear to play such a subordinate part, compared to the internal condi- 

 tions created by growth itself, that for the present they may be neglected. 

 The medium in which growth takes place can never be homogeneous as regards 

 the intensity, or the location of the sources of light, heat, gravity, and chemical 

 agents, so that each part of a growing mass will have its own particular relations 

 to its surrounding medium. The rate of growth, and its character will be variously 

 modified by these local conditions, so that neither a homogeneous body, nor a 

 spherical body could be produced, and, so far as we know, never is produced. The 

 inevitable result is some modification of a sphere consisting of concentric shells 

 or strata, each stratum having various local modifications of its surface. 



If our initial mass of protoplasm begins its growth laden with dead or inert 

 materials, or on the surface of a yolk sphere, diversified local conditions are created 

 at each stage of growth that have a very definite directive influence on each sub- 

 sequent stage of growth. 



It has long been recognized that the presence of yolk modifies -the rate of 

 cell division and the character of the cells produced. But it has not, I believe, 

 been heretofore recognized that: i. Radial symmetry is an inevitable result of the 

 physical conditions created by growth; 2. that the morphological structure of a 

 large class of animals is profoundly modified by the prevailing volume of the yolk 

 sphere over which the initial growth takes place; 3. that the gradual increase in 

 volume of the nutrient surface, accompanied by apical growth, necessarily results 

 in an unlike bipolar concrescence, bilateral symmetry, and a linear sequence of 

 unlike physical conditions, that in turn produce a linear series of unlike organs; 

 4. that the overlying strata formed in a spherical mass of cells, or in a disc growing 

 on a nutrient surface, such as ectoderm, somatic and splanchnic mesoderm and 

 endoderm, are the expression of the various physical conditions successively 

 created by growth. In other words, the whole triaxial framework of any animal 



