264 BIOLOGICAL LECTURES. 



shortening or condensation of the process of development, 

 although this, as Conklin has remarked, does not necessarily 

 signify a shortening of the time of development. I have at- 

 tempted in the preceding pages to follow the various steps of 

 the process, as I believe they must have occurred historically. 



It appears probable that a large amount of yolk affords a 

 condition favorable to the teloblastic form of development, for 

 it renders possible the very early localization of the embryo 

 upon certain portions of the o.^^. In the annelids and mollusks 

 the amount of yolk in the egg is not sufficient to render any 

 such localization of material necessary in the interests of con- 

 densation of development, and a partial localization of material 

 at various points is the most favorable arrangement for rapid 

 development. 



In any form of cleavage which is morphogenetic, it is incor- 

 rect to regard certain cells as becoming functional at certain 

 times. Each cell is functional at every stage of the cleavage 

 and directly connected with morphogenesis. Since this is the 

 case, it is necessary in seeking for the explanation of its vari- 

 ous qualities, such as structure, size, direction, and time of 

 division, etc., to consider, not simply the part which it is to 

 play in the adult organism or in the larva, but the part which 

 it plays at the moment under consideration and at each moment 

 before and after it. Each stage of development is the result 

 of the preceding stage and in some manner the cause — not of 

 the adult or larval organization — but simply of the next suc- 

 ceeding stage. Each stage is a whole, not a series of inde- 

 pendent parts, and, moreover, modifications may occur in any 

 stage which are not connected — except, of course, indirectly, 

 since development is continuous — with the final organization, 

 but relate only to some detail of some particular stage. Thus 

 we may expect to find numerous differences in detail in the 

 solution of the same general problem, i.e., the production of 

 the same general organization. 



I believe that, without an adequate appreciation of the 

 extreme plasticity of the cell, a true conception of organiza- 

 tion is impossible. The cell may be likened to a bit of plastic 

 material, which may be modeled according to the needs of the 



