172 PAUL WEISS 



prevalence of tangential tensions, which, in turn, would cause a large 

 proportion of newly proliferated cells to stay in the germinal surface 

 according to (a). As these additional cells are intercalated, the lateral 

 stretch is reduced and may eventually even be reversed into pressure if 

 the cells begin to crowd laterally; in that case, radial movement (b) or 

 (c) would result. Meanwhile, however, further secretion of vitreous 

 humor has produced further distension so that more new cells can be 

 accommodated in the surface expanse, and the cycle repeats itself. Be- 

 cause of widely varying local conditions, these events will not cause a 

 regular alternation of phases of tangential and radial divisions as in 

 cleavage, but will merely statistically increase the probability of the oc- 

 currence of one or the other. There are some experimental results sug- 

 gesting such a mechanism of growth regulation. When the vitreous 

 humor of an embryonic chick eye is drained, the collapsed retina re- 

 mains greatly reduced in size, but increases in thickness (56). The loss 

 of a progressively distending core would properly account for the re- 

 sult. It has also been demonstrated that when incongruous eye-lens com- 

 binations are produced by heteroplastic transplantation, a mutual ad- 

 justment of size occurs (i, 17). This could be ascribed to the fact that 

 an undersized lens, both because of its smallness and because it will per- 

 mit vitreous body to flow out, will not be able to exercise the distensive 

 function which promotes retinal growth until it has caught up with the 

 size of the eye and sealed the borders. Reciprocally, an oversized lens 

 will produce greater distension, hence, expansion of the germinal layer, 

 resulting in intensified retinal growth. In a similar capacity, distension 

 of vesicles, glands, ducts, etc. by their own or foreign discharges may 

 be a major factor in regulating the extent of their proliferating layers, 

 and consequently of the amount of growth. Perhaps the excessive size 

 of the gut in amphibians fed on a bulky vegetable diet as compared with 

 meat-fed ones (22) is due to the same principle. One could even en- 

 visage that the air in the larval lungs of gill-breathing aquatic amphib- 

 ians is there for a growth-supporting rather than respiratory purpose. 



These examples may suffice to illustrate the dependence of overall 

 "growth rate" on the special mode of growth of each organ, particularly 

 the geometric configuration of its germinal zone. 



Orientation of Growth 



Growth as such, i.e. the mere increase of the specific molecular popu- 

 lation of the cell, is a scalar process. It has no intrinsic direction. Its di- 

 rectiveness is given to it by vector components of the physical frame- 



