142 PATTERNS AND PROBLEMS OF DEVELOPMENT 



the original gradient. But, as noted in these studies, differential dye re- 

 duction gives no information as to presence or absence of regional specific 

 differences. Oxidation-reduction reactions may differ as regards reacting 

 substances, the reactions themselves, and the products formed but may 

 still show the same rates or similar gradients of dye reduction. So far as 

 the results of differential dye reduction and differential susceptibility are 

 concerned, the specific overlapping gradients postulated by Runnstrom 

 may or may not be present. On the other hand, concentration gradients 

 of substance can accomplish nothing in the way of development without 

 metabolism. Moreover, two opposed and overlapping substance gradients 

 may be associated with a single gradient of rate or intensity. The over- 

 lapping polar gradients of yolk and protoplasm in many eggs and the 

 difference in rate of cleavage and other developmental activities at differ- 

 ent levels showing a single gradient decreasing basipetally will serve as an 

 example. Specifically different substances may be present in apical and 

 basal regions of the echinoderm egg at the beginning of development; if 

 not present then, they doubtless are later. When present, they are un- 

 doubtedly concerned in differentiation of cells; but form and proportion — 

 morphogenesis in its larger, more general aspects — appears, at least during 

 earlier developmental stages, to depend to a much greater extent on quan- 

 titative factors of pattern, factors of rate or intensity, than on regional 

 distribution of specific substances. Substance is significant in develop- 

 ment only as it takes part in or affects activities; the activities, not the 

 substances, are formative. Differential dye reduction indicates some of 

 the activities and their quantitative differences and changes of rate but 

 tells us nothing about the substances concerned. These differences and 

 changes in rate appear to be essential factors in development of form and 

 proportion, and they seem to the writer to offer less difficulty in inter- 

 pretation of the more general features of early echinoderm morphogenesis 

 than the specific concentration gradients of Runnstrom. Even if this is 

 true, however, the specific concentration gradients may also be present. 

 Nevertheless, the data at hand seem to favor the view that factors of rate 

 or intensity are more important in determining form and proportions in 

 the earlier stages of echinoderm development than specific or qualitative 

 differences of substance. Definite patterns of form and proportion are 

 evident before cellular and regional differentiations are distinguishable 

 with certainty. Further investigation is, of course, necessary; but in spite 

 of the fact that data and conclusions concerning different species are not 

 in complete agreement as regards presence or appearance before gastrula- 



