7i8 PATTERNS AND PROBLEMS OF DEVELOPMENT 



growth during development, spatial and chronological growth patterns 

 are orderly and definite in axiate organisms. 



Most, perhaps all, growth patterns can be altered experimentally in 

 one way or another: some are altered by autoplastic, homoplastic, or 

 heteroplastic transplantation to other regions, though some growth pat- 

 terns tend to persist under these conditions. Changes in external environ- 

 ment of the whole organism are highly effective in altering growth pattern, 

 as the differential modifications of development show (chaps, v-vii). Al- 

 terations in nutrition, mineral salts, etc., may also be effective. Growth 

 patterns are evidently related in some way to the gradient patterns of 

 development, but the relation is not necessarily direct and simple. 



A characteristic of malignant growths, and apparently an essential 

 factor in malignancy, is lack of a definite organismic pattern of growth and 

 differentiation. More or less differentiation of cells, according to the organ 

 of origin of the neoplasm may take place, but orderly pattern on a multi- 

 cellular organismic scale does not appear. 



SPECIES-SPECIFIC GROWTH RATES AND SIZE FACTORS 



Transplantation experiments, particularly those with amphibian em- 

 bryonic materials, have brought to light certain interesting and significant 

 facts concerning growth rate and size of certain organs in organismic en- 

 vironment of another species, as a few examples will show. When sufii- 

 ciently fed, Amhlystoma tigriniim grows much more rapidly than A. pimc- 

 tatum and attains about double the latter 's size. With feeding up to the 

 maximum intake, optic and limb primordia transplanted from one to the 

 other species show, during most of the larval development, the same 

 growth rate as the normal organs on the donor species and, consequently, 

 become very different in size from the host organs. Before metamorpho- 

 sis, however, the eye and the limb of A . punctatum on A . tigrinum become 

 slightly larger than the normal organs of ^. punctatum.^ In earlier experi- 

 ments it had been found that under ordinary nutritive conditions A . ti- 

 grinum eyes and limbs on A . punctatum hosts grew not only more rapidly 

 than the host organs but more rapidly than those of the donor-species 

 control (Harrison, 1924; 1929a, b). With A. tigrinum as host, growth of 

 A . punctatum organs is retarded. The work of Twitty and Schwind showed 

 that these alterations in growth rate were due, at least in large part, to 

 differences in "nutritive level," that is, to underfeeding of the A. tigrinum 

 larvae which have the higher nutritive requirement. Twitty and Schwind 



5 Twitty and Schwind, 1928, 1931; Schwind, 1931. 



