EXTENSIONS 373 



without feeding but also without intercellular differentiation occurs 

 in some ciliates, as when the large form of Ichthyophthirius pro- 

 duces multitudes of small forms (Mugard, 1948). The beginnings 

 of cellular differentiations may be seen in the formation of mating 

 types or the gametic differentiation of some ciliates, as well as in 

 multicellular stages of certain Sporozoa, the Cnidosporidea. In 

 clonal cell cultures, metazoa are being, as it were, reduced to 

 *' protozoa ". And in Chcetopterus, Lillie( 1906) was able to suppress 

 cleavage of the egg and yet obtained unicellular embryos of fairly 

 normal shape in which the elaboration of ciUa, with a particulate 

 contribution from the nucleus, and imprecise segregation processes 

 led to a fairly recognizable early embryo. Being cytoplasmic con- 

 tinuums, Stentor masses are not multicellular though they do show 

 the emergence of new capacities for morphogenesis. 



Another major point of correspondence lies in the cilia and 

 ciliation. We now know that the basic structure of cilia and 

 flagella are the same in all organisms. Many animals have ciliated 

 epithelia and in these the joining of the cilia by fibrous connectives 

 does not differ fundamentally from that in ciliates. Gruber com- 

 mented on the remarkably similar construction of the membranelles 

 of Stentor and those occurring in the *' corner cells" of certain 

 molluscs, notably Cyclas cornea. Whitman (1893) used this corres- 

 pondence in argument for the inadequacy of the cell theory of 

 development, having to emphasize at that time the neglected and 

 still baffling intercellular organizing relations through which the 

 separate cells, regardless of their size or number, are formed into 

 an embryo. Stentor makes many such membranelles in one cell; 

 a mollusc, one in each of many cells. In their embryonic stages 

 many multicellular organisms are conspicuously ciliated, offering 

 the possibility that something like ciliate morphogenesis may play 

 a significant role in their early development. In the shipworm a 

 silver-staining material is segregated into specific blastomeres 

 (Faure-Fremiet and Mugard, 1948); and in the sea urchin certain 

 cells come to show an argentophile network with apparently a 

 centrosome-kinetosome in each cell which becomes part of a 

 ciliated structure (Mugard, 1957), but there is still no proof that 

 these are causal factors in development. 



Embryologists are generally agreed that development implies 

 an initial cytoarchitecture in the cortex of the egg as a guide for 



