FORM AND GROWTH 139 



(hyperplasia) of cells. This is followed by a second stage in 

 which some cells continue division, while others cease dividing 

 and enter on the period of differentiation and enlargement (hy- 

 pertrophy). In the final period growth is almost entirely by 

 hypertrophy alone. 



VII. Underlying Factors of Morphogenesis 



Finally comes the question as to the underlying factors of 

 morphogenesis. What causes the developmental cell changes 

 associated with growth and differentiation? This problem is too 

 complex and difficult for brief consideration. It may be noted, 

 however, that both intrinsic (hereditary) and extrinsic (envi- 

 ronmental) factors are involved. The intrinsic or hereditary 

 factors are expressed chiefly through the nuclear organization 

 (chromosomes) of the fertilized ovum, but to some extent also 

 in the cell body or cytoplasm. The extrinsic or environmental 

 factors include the essential nutriment, oxygen, heat, light, and 

 other external conditions necessary to development. 



As soon as cell division begins, i.e. } in the morula stage, the 

 mutual influence of the individual cells introduces a new and 

 important set of forces, which may be termed the correlative 

 factors in development. Each cell henceforth is influenced by its 

 neighbors in various ways. There are -physical effects, through 

 spatial relations, contact, pressure and tension, and similar fac- 

 tors. These have been admirably reviewed by D. W. Thomp- 

 son. 49 Then there are chemical effects, doubtless present from 

 the beginning through the products of cell metabolism, such as 

 the "trephones" of Carrel or the autocatalytic agents of Robert- 

 son. Later specific hormones are produced, especially in the 

 well-known system of endocrine organs or ductless glands. Fi- 

 nally the developmental process is influenced by neural effects. 

 Stimuli transmitted by the nervous system affect metabolism 

 and morphogenesis chiefly through functional hypertrophy and 

 differentiation of the cells involved. 



There has been much discussion and controversy over the 



