ONTOGENY IN FILIATES 



85 



development of Teredo norvegica. After the expulsion of 

 the second polar corpuscle, the surface of the egg is homo- 

 geneous as far as argyrophyly is concerned. But during 

 development, certain blastomeres show an increase of ar- 

 gyrophylic particles. These are the two blastomeres X is- 

 sued from D and blastomeres M and Y. They will give rise 

 to the cells of the shell gland, and to mesodermal and mes- 

 enchvmatous cells. Therefore, differentiation of the blasto- 



FiG. 32. Teredo norvegica. Silver-impregnated morula. 



mere is accompanied by cortical events. These events could 

 of course be the result as well as the cause of differentiation. 

 But in the last analysis we are compelled to ascribe differ- 

 entiation to the specific particles of the cell. 



Is it possible to compare morphogenetic factors in ciliates 

 and Metazoa? We know that many parasitic Protozoa 

 show cytoplasmic differentiations in the region by which 

 they feed and that in ciliates nutrition interferes with mor- 

 phogenesis. Many eggs are attached to the ovary; they 

 have therefore a trophic region. This may account for the 

 formation of an axis. We know^ also, thanks to D. M. 

 Whitaker (1940), that in Fiicus the axis may be induced by 

 external factors such as light. 



A. L. Cohen's (1942) principle of self-increasing com- 

 plexity is here very useful. According to this principle. 



