DIFFERENTIAL GROWTH 169 



sense and proliferation are mutually exclusive (ii). From our earlier 

 discussion, it should be plain that the proliferating sources of epidermis, 

 bone, blood, muscle, glands, etc. are differentiated to the extent that 

 each can reproduce only its own kind and none of the others. Yet they 

 are still actively multiplying. Only the full transformation into terminal 

 stages seems to be held up in those cells that are kept in mitotic agita- 

 tion. Proliferation does not interfere with differentiation, but it does 

 impede the elaboration of certain manifest products of differentiation. 

 Conversely, the general impairment of proliferative capacity in cells 

 undergoing terminal specialization can be ascribed to the fact that most 

 of their substance, including mitotic prerequisites, is diverted into the 

 building of differentiation products (e.g. intracellular fiber systems). 



In early developmental stages practically all cells proliferate. With 

 progressive differentiation, however, the outlined segregation into re- 

 productive and sterile groups takes place. Since the latter do not further 

 contribute to the increase in mass, the total growth of a tissue or organ 

 will vary directly with the ratio of reproductive to nonreproductive 

 cells. This ratio in turn depends on the geometric configuration of the 

 proliferating zone and the rate at which newly proliferated cells desert 

 it. Further variability is introduced by the fact that most organs receive 

 secondary additions from foreign sources (e.g. blood vessels, nerves, 

 pigment cells), and, conversely, lose some of their own cells by disper- 

 sion or destruction. Since each organ has its own rule, it is evident that 

 bulk measurements, which fail to separate the actual sources of the 

 measured materials from the "dead weight," can furnish no valid basis 

 for comparing "differential" growth of parts differing in constitution. If 

 percentage increase is faster in one organ than in another, this could be 

 due to either (a) a larger initial proportion of reproductive cells, ov (b) 

 a larger quota of retention of new cells in the germinal zone, or 

 (c) greater infiltration of foreign elements, or (d) less dissipation of 

 cells and cell products, or finally (£') an intrinsically higher rate of re- 

 production in the respective germinal cells. Only differences not ac- 

 counted for by (a), (b), (c) or (d) are to be ascribed to (e), that is, to 

 a higher "growth rate." 



For an illustration let us choose the growing vertebrate eye. Due to 

 its simple form and accessibility it has been a favorite object for com- 

 parative studies on growth, with the increase in dimensions serving as 

 index. How heterogeneous a collection of events this index covers can 

 be readily seen from the following account. The original eye vesicle 

 consists of a certain initial allotment of cells from the embryonic brain 



