THE BIOLOGY OF THE CELL SURFACE 



nevertheless we may say in general terms that each suc- 

 ceeding differentiation is conditioned in part by extrinsic 

 and in part by intrinsic factors, the latter of which are 

 conditioned by the preceding differentiation.^ One great 

 task in the study of the differentiation occurring during 

 cleavage is the analysis of these factors, a task especially 

 difficult because we have no known point from which we 

 may start, for in this respect the egg also must be regarded 

 as an unknown system. 



In the second place we concentrate attention on the 

 cleavage-period in our effort to trace the differentiation 

 of development, because cleavage is a process common to 

 all animal eggs. Every species of animal egg passes through 

 cleavage, but not every animal Qg^^ e.g., of sponges and 

 of coelenterates, passes through the three other periods 

 enumerated above. Then the differentiation that takes 

 place during cleavage we regard as that embracing in 

 a wider sense those characteristics common to animal 

 development and most readily reducible to general terms. 

 As we shall later see, whatever the period of development, 

 in which differentiation reveals itself to us, the mode of 

 differentiation is always the same. This being true, study 

 of the differentiation in the most generally appearing stage 

 has distinct advantage. 



In the third place we have the practical reason that, as 

 experience has taught, of all the periods of development the 

 cleavage period lends itself most readily to resolution of 

 the processes into closely set stages; thus by experiment it 

 is possible to analyze the factors which set up the conditions 

 for differentiations in a more normal or natural manner 

 than, for example, in experiments with transplantations 

 involving conceptions of "organizators" and the like. 



1 Cf. Delage, 1S93, PP- 765-766. 



2go 



