CLEAVAGE AND DIFFERENTIATION 



each spermatozoon moves by the lashing of its tail. If the 

 spermatozoa are caused to form balls by the addition of 

 some substance which agglutinates them with their heads 

 sticking together, the tails now beat one after the other 

 in succession. It is also well known that cells of a tissue 

 when isolated and grown in tissue-culture manifest form- 

 changes and behavior which never occur when they are 

 part of the tissue.^ Finally, blastomeres when isolated 

 reveal behavior arising out of their isolation. In brief, 

 therefore, the behavior of blastomeres during cleavage 

 must be in part due to an integration brought about by 

 the intercellular connections arising from the ectoplasm. 



The movement of blastomeres is an undoubted factor in 

 development and this movement is a function of the ecto- 

 plasm. The rise of cells out of which is formed an embry- 

 onic area in a specific region is to be attributed to a 

 movement on the part of the cells which is closely akin to 

 amoeboid movement. True, physical factors may be con- 

 cerned in the displacement and new alignment of blasto- 

 meres, but these are to be regarded as subsidiary. No 

 purely physical theory has as yet accounted for the process 

 of invagination by which a hollow blastula is converted into 

 a gastrula. On the assumption, however, that cells take 

 up a certain position with reference to others through their 

 active movement we may approach the explanation of 

 many processes, as invagination and evagination, epiboly, 

 etc. 



Finally, there is clear experimental evidence that during 

 cleavage the ectoplasm reveals structural changes and 

 activities which parallel cleavage-rhythms and which indi- 

 cate that the ectoplasm plays a role in determining the 

 direction of differentiation. Some simple observations of 

 my own may be cited, ^ 



^ Harrison, Lewis and Lewis, et al. 

 - Just, 192SC. 



335 



