CLEAVAGE AND DIFFERENTIATION 



clearly and so beautifully shows the quickly occurring 

 changes in ectoplasmic activity. 



The ectoplasm stands not simply as a barrier of the cell 

 against the outside world; it is also the medium of exchange 

 between cytoplasm and environment. As such, it is the 

 first cell-region to receive impressions from the outside 

 world; through its delicacy of adjustment and fineness of 

 reaction, it constitutes the first link in the chain of cyto- 

 plasmic reactions and sets the path for the orderly suc- 

 cession of events comprising the course in the difi"erentiation 

 of development. 



The moment to moment changes in the ectoplasm in 

 its response to the cell's milieu set up conditions in the 

 cytoplasm which are favorable to the mechanism of the 

 synthesis of nuclein, i.e., of the building up of nuclei out 

 of cytoplasm. This synthetic reaction constitutes the 

 removal of a barrier to those cytoplasmic reactions leading 

 to differentiation. Alongside this function of setting up 

 conditions favorable to the releasing-reaction of nuclear 

 syntheses, is the growth of ectoplasm through the transport 

 of ground-substance to the cell-surface. The ectoplasm 

 impresses the cytoplasm, and this ectoplasm-induced cyto- 

 plasmic activity brings about the nuclear behavior which 

 underlies genetic restriction. Ectoplasmic behavior is thus 

 the direct and so far the only visible manifestation of the 

 cause of the differentiation of development which takes 

 place during cleavage. 



339 



