44 The Unity of the Organism 



which are the most indubitably hereditary are those which 

 distinguish the smaller but yet definite taxonomic groups. 

 A little consideration will convince one that about the most 

 crucial cases would be those in which the development of 

 differential attributes could be traced directly to cell struc- 

 ture and development. It so happens that vast as is our 

 knowledge of histogenesis, the part of it which answers 

 directly to the requirements here laid down is by no means 

 large. 



The S pinnies of the Ascidian Genus Styela 



The best instances I have been able to find are super- 

 ficial appendages in some animals and plants, so small that 

 they consist of a few cells or even of a single cell. One strik- 

 ing instance of this sort has come to light in my own studies. 



FIGURE 44. SPINULE CELL OF STYELA YAKUTATENSTS (AFTER HUNTSMAN). 



n., nucleus. 



It is that of the minute spines which cover the inner surface 

 of the branchial siphon in some species of the ascidian group 

 of Styalids. Huntsman first called attention to the fact 

 that each spinule is a single cell, and that at least in some 

 cases the structures furnish differentiating attributes for 

 species. Miss Forsyth and I have reexamined the point for 

 Styela montereyensis and S. yakutatensis, and are able to 



